Tuesday, September 20, 2011

Micro Battery Off Grid System

Sorry for the lack of posts...  No excuses, but I have been busy over the summer [Damn. That sounded like an excuse...]

Remember the Big Capacitors of a few posts ago?  Well they've found a use :D

They're really cool, as it happens.  I charged them up and left them with a Volt meter on them, to see how long they could hold a charge for.  Kinda expected they would self discharge in a few minutes, if left alone.

Time ticked on, and they didn't seem to discharge (now that the LED Voltmeter was not connected to the tops).  In fact, a couple of weeks later they had only discharged down to a few Volts less than when they were full!  Extremely low leakage current.  When I shorted them out by accident, they still packed a terminal blackening punch :D

I got a couple of big 230Wp panels to test from a friend, and got to wondering about how I could use them.  I had a spare 1kW inverter and a spare MPPT charge controller.  Having a big 500Ah battery bank on the main house, I wondered about the opposite.  How small a battery could you use for an off grid system..?

Luck came in the form of a bargain bin at a DIY superstore.  They were selling off loads of drill batteries for just £3 each.  [rant mode ON] The cunning manufacturers change the drills and batteries to be slightly different each year so that your old batteries can't be used on their new drills... (built in obsolescence, again).  It also mops up those pesky consumers who think that they can keep their old drill until it breaks... Oh, no.  They change the shape of the new "spare" batteries so that they don't fit the old drills - forcing you to buy a whole new drill set.  [rant mode OFF]
Anyhoo... it means experimenters can pick up brand new 12V and 14.4V NiCd battery packs for less than the cost of a single cell at an electronics store.  These cells are also rated for high current applications - A cordless hammer drill can pull 20A when working hard.

So, I had a pair of 12V drill batteries wired in series to make a titchy (for off grid use) 24V, 1.5Ah battery pack.  But it can deliver 20A easy (480W).  Good for a steady load but not enough to start something big, like a fridge or big TV...  Enter the big capacitors!
The capacitors can pack a punch of maybe a few hundred Amps for a second or two and the NiCd batteries can deliver 20 Amps for about four and a half minutes.  NiCd cells are good in that very small ones can sustain high drain currents without their effective capacity being degraded.  Small lead acid batteries would be useless, as firstly their capacity drops as you drain them faster and draining them until they are empty is a sure way to kill them.  NiCd cells don't mind being drained flat - it's what happens when they're used in a drill.

So what I had was a 460W solar array with a 480W four minute "buffer".  You can use this system to power quite big stuff on a sunny day without the bulk and expense of a big lead acid battery bank and it will keep your load going when small clouds pass in front of the array.  It only takes about 15 minutes of charge for the battery to get to full from empty.

Some drill batteries (the ones designed for a fast charger) even come with a temperature sensor in the pack, so the drill charger won't over charge the pack and cook it. Unfortunately, the 12V packs I bought didn't, but the Morningstar MPPT-15 charge controller has one.  I super-glued it to the top of one of the packs.  This charge controller is also fully programmable.  So, despite being primarily for lead acid batteries, it can easily be programmed to charge NiCd batteries.  You just have to set the Voltages differently (a topping charge of 30V and then a float of 28.5V) and the timers much shorter (as the pack is so small).  I programmed the temperature compensation to do nothing below 25 degrees C, but then start an aggressive negative slope above that.  So, if the cells are heating up from high charge or discharge currents, the charge Voltage would be reduced.  The charger itself is limited to 15 Amps charge current.

So far it's worked ok.  I managed to run a 140W 28" LCD TV on it for a whole afternoon with only a couple of power failures (when it started to cloud over in the afternoon).  The fully charged battery could run the set for about 12 minutes without direct sunlight on the array.  The capacitors provide the oomph to start the inverter and the TV without the Voltage dropping. At the "float" Voltage of 28.5V, the two 1F capacitors store 28.52 x 0.5 Farads x 0.5 = 203 Joules of energy - or enough for 203W for 1s or 406W for 0.5s or 812W for 0.25s (although that would require draining the capacitor flat).

Huge battery banks are hugely expensive in off grid systems.  This alternative method allows quite moderate loads to be run in good weather, during the day, with only a tiny cheap battery.  If you planned it right, you could use the inverter in the day to do power hungry things, and then just use the NiCd battery pack at night to run ultra low power 12V DC LED lights...  Or watch a very short TV show :D

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