The CellLog8s LVD is fine and works well at protecting the battery bank from over discharge but it doesn't give you any warning that the lights are about to go out.
So I started thinking about some kind of pre-alarm that would give me a few minutes warning (under heavy load) that the battery was nearly depleted. I could then turn off some big loads and buy some time.
The old SmartGauge Voltmeter has a programmable relay in it too. But it only works on pack Voltage. I've been using it as an obvious visual reminder of pack Voltage (it's no good at reading SoC for non lead acid batteries).
Then I started looking around the house for something to use as a buzzer or alarm sounder that the relay could activate so that you'd get the message...
I found a small toy sound effect thingy that you press the top and it makes a cool police car sound. I actually found another one that my wife had that made steam train noises and in fact a whole load of similar sound chip enabled things, like a Dr Who Darlek bottle opener that says "EXTERMINATE!" when you close a circuit (with the bottle top) and a Christmas card that plays George Michael's "Last Christmas"...
But I decided to go with the police car :D
Taking it to bits was very easy and then all I had to do was solder wires on to the existing switch contact and run these out to the alarm relay on the SmartGauge and program the chosen low Voltage alarm.
The toy still uses the same two 1.5V button cells to make the noise. The SmartGauge does not provide any power to the relay contacts so external power for the alarm or whatever is needed. If the batteries in the alarm go dead, it does not affect the safety of the battery bank as this alarm is just for information.
When the relay contacts close, the police siren only goes off once for a few seconds and then stops. This is a good thing! Saves the batteries in the alarm and prevents bricks being thrown at the thing for sounding too much once the message has gotten through to whoever is within ear-shot of it :D
With little prospect of the firmware being fully fixed, I decided to implement a work around to make the CellLog8s at least work as a "one shot" Low Voltage Disconnect (LVD) for the inverter.
The problem was that without proper hysteresis in the CellLog8s firmware, the alarm output would flip-flop in an unstable way near the alarm set point value. So I had to devise a way to iron out this transition behaviour and make it trigger once only.
I found a little DPDT latching relay in Maplins that does the trick, but I had to rebuild the interface board that I'd made previously. In the video you can see the new circuit.
In this new version, the inverter receives an "Enable" signal from the interface. This just connects to the common pin on the Remote/Off/On select switch on the inverter front panel. The new relay is stable in both positions of its double throw output and has two coils, one to select each output mode. It only needs a single short pulse to cause the state change and then further pulses have no effect (as you have to energise the opposite coil to change the state).
So, you press a button to "Enable" the inverter (or reset it, if it had tripped). This just flips the relay "on".
The 680 Ohm resistors in series are because the relay has 12V coils with
a measured DC resistance of about 700 Ohms. They weren't quite equal
though and (by luck more than judgement) I happened to pick the coils in
such a way that the alarm state coil is the "stronger" one, so that
when the alarm state is "true", the "reset" button does not work...
Useful that. You can't force the inverter to start up when something is
wrong.
The second pole on the relay is just used for the LED indicator.
The output of the CellLog8s alarm port (now set to Normally Open) sits and does nothing until the set point is reached, at which point it will trigger. The alarm port goes to closed state and triggers the "Disable" coil on the relay. The LED goes out and the inverter is forced to shut down. It cannot restart until the alarm condition has cleared and the reset button is pressed on the CellLog8s interface (and of course after you've investigated why it tripped!).
As programmed in the CellLog8s now, either a pack LVD or a cell imbalance alarm can cause it.
Next, all I had to do was hack the inverter to accept the Enable signal...
Here's another video of me "hacking" the inverter to get at the switch on the front panel and wiring in the connection to the new interface. A bit of testing, too.
Now the battery is fully protected from any low Voltage drain from the inverter (the main load).
The advantage the new system has is that the relay consumes no power to hold the inverter in the enabled state. Just a pulse of current from the reset button and then nothing.
In the alarm state, the other coil consumes 20mA for as long as the alarm is triggered. In practice, the load from the inverter is usually such that the pack or cell Voltage sags to the limit and triggers the alarm. Instantly, the load is disconnected and the pack/cell Voltage recovers enough to rise above the alarm set point, which cancels the alarm. Now the relay consumes no power again but is latched in the "Off" state.
In theory, the charge controllers, the SmartGauge, and even the CellLog8s itself could cause the pack to drain down and be damaged. But as I've set the cut-off Voltages quite high (24.0V pack and 3.00V per cell), it would probably take several days with no solar charge (the PV disconnect breaker thrown) to drain the last few Ampere.hours from the pack and damage it.
A bit more tinkering with charging settings, increasing the float Voltage again to 27.00V (3.375Vpc) seems to null out discharging with the long sunny afternoons and water heater running.
I also changed the timers a bit. If there was not much load on the battery for a few days, even charging for 10 minutes per day to the upper Voltage limit might start to cumulatively over charge the cells. So I'm now making use of the absorption extension timer.
If the pack Voltage never drops below 26.60V (3.325Vpc) during the night, the next days absorption timer is set for only 1 minute top charge. This effectively eliminates the possibility of cumulative over charging. If we went away for a long time, I'd shut the whole system down.
If "normal" amounts of charge are taken from the pack, the Voltage drops to under 26.60V and the next days absorption timer assumes an extended absorption timer setting of 20 minutes.
I had noticed that 10 minutes wasn't resulting in a very large fall-off in input power to the battery during the constant Voltage phase, so extending the time seemed appropriate (with the new safeguard of a much shorter default timer).
The small SunSaver 15 Amp charger has now had its absorption timer defaulted to 1 minute for any condition.
With the clear blue days we've been having, I've seen total charge rates as high as 72 Amps, and at the top of charge, the battery pack doesn't need the assistance of the small charger for long before the Tristar charger can hold the constant Voltage.
Some general tuning and tinkering over the last couple of days.
After watching the battery pack charge and float, I noticed that it was starting to discharge a bit more than I'd like when floating. So I increased the float level by 0.1V to 26.90V and then observed the next charge day. This time, rather than discharging at 3 Amps, it settled into a discharge of around 1 Amp.
Meanwhile, things are moving forward with the CellLog8s problem and development of an interface to my inverter for low Voltage disconnect.
Junsi, on the OEM RC Groups thread, managed to replicate the unstable alarm port output problem in his lab and set about looking for a remedy for it.
Not 24 hours later, I received a PM on the forum and then a new beta firmware code to test! Now that's FAST.
Uploaded the firmware (v2.09) to the Cellog8s, now connected to all the cells in the pack, and played about with the battery until 3am to see how it was now.
Much better, is the answer. But still a ways off being useful without bodging some external electronics to fix the remaining problem...
At least now the alarm triggers reliably when some way above / below the set points. But there's still a lot of instability at the set point. The alarm trigger has no hysteresis in it. With a big battery you get VERY slow changes in Voltage and then the battery can spend a long time transitioning across the set point (and I mean a few minutes spent dropping the pack Voltage by 1-2mV at a 150W load!).
Anticipating that they would fix the software, I built a follower relay
module (that just follows the sense of the alarm output of the
CellLog8s). The relay itself came from an old broken mains timer switch and was convenient as it had a 24V DC coil.
The instability of the CellLog8s alarm output made the relay chatter noisily near the set point, with the transition instability.
If I used it "as is", it would probably do what Jack Rickard's VDR (voltage dependent relay) did to his test load and A123 battery pack. The load and pack cycled on and off furiously at the switching set point, and then both of them exploded with the stress of a few hundred Amps being pulsed rapidly.
The addition of a programmable variable for alarm set point hysteresis would eliminate this problem. If you have a low Voltage alarm trigger point at 24.00V and hysteresis of 0.5V, then the alarm will trigger ONCE at 23.99V, and then stay triggered in the alarm state until the pack Voltage rises to 24.51V.
With no hysteresis, your alarm triggers multiple times as the Voltage floats around the 23.895 to 23.995 zone, in exactly the same way you see a DVM last digit toggle randomly between two values when it is close to the threshold of the next digit. Fine for a DVM display (even desirable as you can interpret the toggling as meaning the value is very close the the transition point)... VERY BAD for a load controller.
First day of solar charging the new Winston Battery pack.
Got the CellLog8s wired up into the individual cells during the night, ready to monitor them for any over charging.
It was quite simple in the end. I'd thought about making lugs from copper strip. I'd thought about soldering wires on to the cell terminal straps.
In the end, all I had to do was splice on thin stranded bell wire to the 9 way header cable (cutting off the other plug), and then insert the thin stranded wire into the laminated cell strap sandwiches. Each strap is made of several copper plates held together with heat shrink tube (and the terminal bolts of course). When bolted back down, the wires are securely held and aren't going anywhere. Well, maybe at least until one of the cats decides the spaghetti is a toy and pulls it out or trips over it! I wrapped the cable in some spare cable tidy spiral-wrap stuff. I might shorten the leads later but was toying with the idea of having the monitor mounted somewhere above the battery pack.
Got the two Morningstar charge controllers re-programmed with very conservative settings to start to get a feel for how the pack would behave. It was very sunny from the get-go, with the chargers putting out a combined 55 Amps into the battery. No magic smoke ensued. For that matter, nothing even got the slightest bit warm; none of the cables; the new "200" Amp battery disconnect breaker; any of the cells.
I checked the live data feeds from the two chargers with the MSView software to double check that they were adhering to the expected target Voltages and temperature compensation slopes. Some months ago, I'd previously had an issue with one of the programming wizards that had a bug in the temperature compensation settings, but Morningstar were quick to provide a bug fix after I mailed their software support team.
Basic settings are as follows (per cell values in brackets):
Absorption Target Voltage: 28.00 (3.50)
Absorption (Constant Voltage) Cumulative Time : 10 mins
Absorption Time Extended: none
Float Target Voltage: 26.80 (3.35)
Float Exit Time: 12 hours
HVD (trigger): 29.00 (3.625)
HVD (release): 26.40 (3.30)
Max Regulation Voltage: 28.40 (3.55)
Temperature Compensation: Pack -60mV/C (25C to 80C)
The smaller SunSaver charger has the same settings with the exception that the absorption target Voltage is set to 27.90V, so that this charger quits before the main one and then the main one is in the driving seat to control the finishing charge. This is especially important because the SunSaver charger does not have remote terminal Voltage sensing.
An overview of the programming wizard in the MSView software is included in the second video below. Speaking of which, today's video is a monster. It was so long (23 minutes) that I had to split it into two as YouTube complained it was too long. Soon I'll be making blog videos almost as long as episodes of EVTV :D
And so the initial charging comes to a conclusion (finally!) and it was time to put the battery pack together in its new home, behind the sofa :D
Here's the video.
Not many photos, as I got carried away with video recording each step.
First problem was to fix the M14 cable lugs on the positive and negative battery pack flier leads that would go to the existing M8 stud terminal block.
Without the aid of a £200 industrial crimper to crimp the 120mm2 lug on to the 35mm2 cable, I resorted to using one claw hammer as an anvil and another to beat the sleeve flat on the copper. I folded the end of the cable over in two, so that it was chunky enough to fit snugly in the sleeve. Then I used my plumbers Pipemaster soldering iron to heat the sleeve up and silver solder the cable in place to guarantee a good connection.
I also soldered a couple of small wires into the lugs to use for the Smartgauge, the charge controller remote Voltage sensor and the CellLog8s logger.
Then it was just a case of using a bit of fine wet and dry sanding paper to clean off the oxides on the cell terminals and the copper link straps and bolt the thing up. I held the wrench close to the socket so that it wasn't possible to apply too much force and only really tightened the bolts to the point where the spring washer was sitting flat and gripping the straps. It's not like this pack will be bouncing down the road in the boot of an electric car, so there's no worry about vibration loosening the terminals.
I was more tense than usual when doing this bolting up. These cells can muster 2,400A output without really blinking. A ring or watch strap or 1/2" socket wrench with no insulation on it could make for some impressive fireworks. Be afraid... Be very afraid. It's not the Volts that kill you, it's the Amps.
And so a while later... Actually, a lot later, because my mate James came round for the afternoon, I got round to finishing up the sanding, bolting up and wiring. Here she is:
And a close up of the terminal block with the addition of the old home made shunt in the negative battery leg (the horseshoe shaped bit of 35mm2 cable).
The shunt is pretty accurate (well good enough to see what's going on) and using an extra 8mm stud gives me the flexibility to change the shunt for something else... Maybe a real shunt for a proper amp-hour counter or just a better home made shunt (now that I have an 80,000 count DVM to calibrate it with).
Just clipped the CellLog8s on the battery pack plus and minus to see what it would do with the full 27V on it. Tomorrow I'll make up the special 9 pin lead to connect it to the individual cells.
I also programmed the two Morningstar charge controllers with my custom settings for charging this unusual battery bank. More on that later...
The battery was at a high state of static charge after its initial charge. It sat at 27.59V with no loads on it yet. Turning on the inverter and loading it up to 35A briefly, brought the pack Voltage tumbling down to a steady 26.53V. Releasing the load saw it recover to about 26.75V. I make that about 6 milli-Ohms for the whole pack, including the cell straps terminal connections!
While lugging the cells around for their initial charge, I noticed something odd about them. Three of them had free liquid electrolyte sloshing around in them, while the other five appeared to be free of the free liquid (if that makes sense). That is, some cells appeared totally solid when you picked them up and tilted them, while others made a sloshing noise of liquid running from one end of the cell to the other.
Two of the ones I'd already charged had this liquid, but one that was waiting to be charged also had some liquid, so they came out of the crate like that...
So I weighed all the cells to see if the "wet" ones were any heavier than the "dry" ones. Maybe they'd been overfilled? But no, they were all the same weight to within 100g and some of the dry ones were heavier than the wet ones.
I mailed GWL, and asked them about the liquid and whether it meant that I should use the cells only in an upright attitude. I know lots of people have mounted them on their sides in cars and the cells are often promoted as being "sealed" and suitable for mounting in any orientation except upside down.
The reply came back that some free liquid is normal in new cells. It may have been evolved during the pre-charge at the factory or in pre-delivery "testing". But the liquid should all be absorbed back into the cell plates and the cell become "dry" over a number of charge and discharge cycles. Over charging or over discharging the cells will lead to lots of liquid and gas being evolved, which would cause the cell to swell up and vent. If the cell is on its side, it could spray the liquid out of the vent. If it's upside down, it will almost certainly spray any liquid inside, out.
So... Nothing to worry about and the recommended installation position is "right way up", to allow emergency venting without loss of liquid in the event of serious fault or abuse of the cells.
After the previous experiments with the CellLog8s alarm output, I posted a query on the RC Groups forum where the manufacturer provides support for the range of Junsi chargers and cell monitors.
They looked at my video and blog entry and suggested that the problem might have been a grounding problem in my wiring of the LED to the alarm port. The alarm port negative needs to be referenced to the cell negative.
So here's my update on the situation. I wired the LED +ve into the cell +ve terminal. The negative of the LED goes to the +ve alarm port and the -ve alarm port goes to the cell -ve terminal. The chargers are all connected to the same points too.
You can see the LED and its connection points in the photo above.
In the photo below you can see the problem.
The alarm was set to trigger when the pack Voltage exceeded 3.65V. It's now 3.95V as shown on the charger and the logger display is showing 3.973V, alternating with the alarm status "over". But at the same time, it's not beeping an alarm and the LED connected to the normally closed alarm output is ON (indicating no alarm condition).
Here's today's video showing the erratic behaviour of the alarm port and beeper.
When no.4 got to be full, I decided to play with the CellLog8s and its alarm output. I'd noticed that when it gets to (and beyond) the set point of the over Voltage alarm, it would flash "over" on the display but not always beep. It goes though random phases of beeping every 4 seconds (like it should) and then not beeping for a while.
So I set it up with the provided alarm output cable (with a tiny plug and tiny thin wires...) to a 12V LED. This has the current limiting built in for use as a panel lamp in cars. You can see the alarm port on the device and the external LED circled in green in the photo.
The plan is that the CellLog8s will be my low Voltage monitor, both for the pack as a whole and individual cells (as any cell that goes below 2.0V will be permanently damaged).
The 3kW inverter is my only load and has a LVD cut-off built in, but there are two problems with this. The first is that it has a fixed cut-off Voltage of 21.0V, which is too low. It's 2.62V per cell. For high drain applications (0.5C / 200A discharge) 2.8V is the recommended cut-off. For lower currents, the cut-off Voltage is actually higher. The cell can be considered "empty" when it gets to 3.0V. This means that for the many hours of a day where the inverter is drawing a mere 3-4A while doing not a lot, the worst case applies and I need to shut the thing down when it gets to 24.0V pack Voltage.
Then it's also possible for the pack to be out of balance and one cell get below 3.0V before the others as the pack nears "empty". If I accurately bottom balance the cells, this shouldn't happen but I want to catch it if it does.
The CellLog8s does both of these things. It monitors each cell Voltage, and it monitors the pack Voltage. And the alarm output can be triggered at any programmable level.
Now, back to the test... Because I'm charging the cells, I set the over Voltage alarm to 3.65V so that as the cell gets near to the end, it would alarm so that I could watch it finish and shut the charger down. Just to see the alarm work.
Ideally, the alarm output would trigger and latch. That way, if the threshold is crossed, the load will be disabled and then require manual reset before it could be enabled again.
Anyway, for the test, I had the alarm set to "Normally Closed" output. This means that the LED comes ON when there is NO alarm condition. This would mean the CellLog8 has power (to drive the output transistor) and the wiring is working. This would provide a fail-safe "inhibit" signal to the inverter remote port. When there is no alarm, it's safe for the load to run.
When the alarm is triggered (by low Voltage), the LED should turn OFF. This would signal to the inverter that there was either a low Voltage alarm or that there was a fault in the CellLog8 (open circuit wiring or no power to the device). In either event, the inverter should be disabled or shut down.
Well, as you can see in the video, it sort of worked. The alarm was triggered at the appointed Voltage and the LCD display started flashing "Over" to tell me what kind of alarm it was. The CellLog8 started beeping (as it should) and the LED turned off. But... It then came back on and randomly turned on and off.
At first I thought it was a hysteresis problem (with the alarm threshold being crossed multiple times as the Voltage crept up) but with the cell well over the limit, the LED continued to randomly turn on and off. No good.
Time to post a bug report on the RC Groups forum where the manufacturer hangs out. They've been quite good at listening and producing bug fixes and new features for the device firmware but this is a pretty basic problem that should have been ironed out by now.
Failing that, I could work around the problem with an externally latching switch / relay, but if the software on the logger worked properly in the first place, it would reduce the interface complexity and so the number of points of failure.
While charging the first cell, it quickly became clear that the 5A output of the bench power supply wasn't enough to get charging done in a sensible amount of time. The cells appear to be delivered with 50% charge and so I have to put another 200Ah into them. At 5A, this would take 40 hours at least.
I'd also had some concern expressed by Jack Rickard about such a slow charge rate being effectively a trickle charge that might not even be able to push the cells to the top and could even overcharge them in some way by trying for too long.
There was some conflict in advice, in that Jack has been using these cells for a couple of years and has never done an initial charge on them and never charges them to 4.00V. His method is to stop charging at 3.60-3.65V (but often charging the cells in series), safely under charging them, with no active BMS present (or required). I take his advice seriously, as he's tested lots of cells in his lab and blown up a few when over charging them on the bench!
However, the Winston Battery operator's manual is quite clear (and insistent) that this initial charge to the full 4.00V is done before the first discharge. GWL echoed this in their user FAQs. So I contacted the Technical Manager at GWL for advice on whether I should do the initial full charge, whether the 5A bench PSUs are suitable and whether any harm comes of doing the initial charge too slowly. The Winston Battery manual recommends a charge rate of between 0.1CA and 0.5CA, but this is out of my reach as it means a PSU that can deliver between 40A and 200A!
The response from GWL was go with the full 4.00V CC-CV cycle and it's ok to do it slowly, so long as you terminate the cycle properly when the current has reduced in the CV phase. They have posted a technical paper (PDF) warning about using shunt BMS systems that hold cells for too long at the terminal Voltage (4.00V). However, this was in the context of holding cells at that Voltage for long (cumulative) periods after the charge acceptance current of that cell has effectively reached nothing. Not the same case as doing a very slow charge and terminating at the end of charge acceptance of the cell.
So, on balance, I'm going with the advice of the manufacturer and the supplier of the cells on this. At least that way, if it turns out to be bad advice, they are on the hook for giving it to me - if it comes to a warranty claim.
I'm using these new 80W constant power SMPS PSUs from Maplins. They can run in parallel with a master/slave link and they also have a remote sense input so that the constant Voltage dialed in is measured at the cell terminals and can compensate for the Voltage drop in the heavy current power leads. This way you dial in 3.97V and you know the charger will hit the mark.
The unit is small, light and efficient. Doesn't get hot in use, has no need of a fan and is power factor corrected. At £100 each, they're not cheap but also checking RS, Farnell and Rapid didn't turn up anything better value.
After the initial charge, the manual advises that it's ok to subsequently charge within any part of the cells safe operating range from 2.50V to 4.00V. So I'll adopt Jack's strategy of making sure all the cells are bottom balanced (at 2.75V when "empty") and then never fully charging them, to avoid the problems and need for active top balancing and current shunting and all that expensive and failure prone junk.
The CellLog8 should be all I need to prevent over discharge of the pack or individual cells in the pack. It can monitor each cell individually and I can set an alarm for any cell that goes below 3.00V or the whole pack goes below 24.0V. It can also alarm if any cell differential Voltage (the difference between individual cells) exceeds a limit. This will tell you if the pack is out of balance -even before it gets to being near full or empty. The CellLog8 has a beeper and an open collector transistor output that can be used to provide an "inhibit" signal to my inverter to shut it down until the solar array can recharge the pack.
Of course, the required 9 pin header cable for the CellLog8 didn't come with the thing, so I had to source that from a radio control models shop (Electriflyer). They also sell the Junsi lithium battery chargers that use the same JST-HX balance lead (model BW-9-11). Just under £5 delivered.
The cable is actually meant to connect a Junsi iCharger 208b charger to a balance board (hence the 11 way header on the other end) but you can either cut off the big plug or use it. The CellLog8 doesn't have a proper JST socket so a plug with more than 9 pins will fit; the spare ones will stick out over the end of the 9 pin header on the device. I might cut the lead in the middle and then have two usable plugs for the gadget.
Another thing I had to order was some terminal lugs. These are also a bit harder to track down because they are M14 stud holes. I found some on eBay from Clarik Engineering Supplies. They take up to 120mm2 cable (too big for my 35mm2 cable). I might have to use a blow torch or my pipemaster plumbing soldering iron to attach cable to them. These alone were another £7 delivered... Everything is expensive when you have such massive terminals on a battery!
The Winston Battery cells are expensive. This lot of 8 cost me an eye-watering €4096 which translated into £3540 including
20% VAT 2% credit card payment surcharge, 2.5% currency exchange charge
(from VISA as it was in Euros to Pounds) and TNT overland shipping
(quite quick at 3 days from Prague). At least when I put the payment in
it was the week when all the Euro countries got their credit ratings
downgraded and the Euro slumped against Sterling.
Pretty steep, but
actually a lot cheaper than good OPz solar cells. A quick check on the Tanya battery web site shows "12 OPzV 1400 Sonnenschein A600 Solar Battery" cells of 1,400Ah (C100) would cost £822 each (inc VAT but excluding delivery). That's £9,864 for 12 cells to make an equivalent 24V sealed type of lead acid battery pack that might last 10 years and be usable indoors (not a flooded fork lift type battery).
Even the Deka Solar gel batteries were quoted to me at £250 each - £1,000 for 180Ah (C5). And they only lasted 850 cycles or 2.5 years (although they were 2nd hand when I got them for £50 each, so I can't complain).
Then there's the problem of weight... the A600 bank of cells weigh 97kg each... 1,164kg in total. By comparison, the Winston Battery cells only weigh 14kg for a total pack weight of 112kg. Even boxed as a whole pack it would be an easy two-man lift.
Just starting to play around with the video recording on my Samsung Galaxy SII phone. So, here's my first video blog entry for this lithium battery project... Enjoy!
Each cell has a factory date code and serial number printed on it. All the cells I have are in serial number order from 6288-6295 and were all made in October 2011.
I ordered some cell connecting straps with the cells that come with the A2 grade stainless steel M14 2.0mm pitch thread terminal bolts. You have to order the straps as the cells don't come with bolts. The sets also include a stainless steel washer and a spring washer.
I only ordered 7 straps (as that's all you need to connect together 8 cells) but the kit rather unhelpfully doesn't include the two additional bolts you need to make the end plus and minus terminal connections. So I had to source these from a fastening specialist in the UK (Westfield Fasteners).
I ordered:
2x A2_Screwbolt_HexHd_M14_20_THRfull (M14 x2.0 x 20mm fully threaded hex head set screw bolt)
2x A2_Washer_FormA_M14 (large diameter M14 washer - there are 3 kinds of washer)
2x A2_Washer_SprgSq_M14 (a square profile spring washer)
This was £8.75 delivered.
The cell straps themselves are a bit comedy. They're made from multiple copper plates that have been stamped out and then tied together with heat shrink tube.
The other thing I ordered was a Junsi CellLog8 cell logger. This is a tiny device that is designed specifically for monitoring of multi-cell battery packs. It can do lead, NiCd, NiMH, and all the various types of lithium cells.
What it does is monitor the individual Voltage of each of up to 8 cells (handy as I've got just that number of cells) and then you can set alarms for over, under, and differential Voltage limits on each cell and the pack as a whole. It will beep on some / any of these conditions and also drive an open collector transistor output for a relay or whatever (either NO or NC mode). This is going to be one of the key battery protection elements of my plan.
Well, we're back from our adventures in Japan and while out there, I made an important decision...
I hate lead acid batteries. They're rubbish for solar applications.
So I took the plunge and put in an order for some batteries I've been eyeing for a few months now. They get used a lot in the DIY electric car (EV) arena because they are light, powerful, small and can withstand deep discharges daily. What's more, they are based on lithium ion technology and so do not need or even particularly like being kept at full charge all the time, as lead acid batteries do.
In short, they are much superior to lead acid batteries in many respects (but not all).
Of the makes available, the only one that was available in Europe (let alone the UK) was the Winston Battery Company make. These are a new sub-type of lithium ion cells that use yttrium (a rare earth metal) to significantly improve the reliability and safety of these cells, compared to the previous generation of cobalt based lithium ion cells. These could explode violently in a ball of flames (hence the stories in the papers about exploding mobile phones and laptop batteries).
Over the next few days and weeks, I'll be getting to grips with these new cells in order to build them into a 24V nominal 400Ah battery pack for my solar system.
