Friday, December 9, 2011

New Battery Time (Again)

Well, the Deka Solar gel batteries finally caved in.  They've been in service daily since about October 2009 and they were second hand when I picked them up.  After another 2 years and 1 month (some 760 charge cycles), they're way down on capacity and have gotten very high internal resistance but have more than met their expected life.

When pushed hard (say by the 2.2kW heater in the washing machine), they could still pump out 20-30 Amps with the other set of batteries but they took a long time to recharge, only managing to absorb 9 Amps of charge.  The other bank will greedily suck up nearly 40 Amps, even through its deliberately under-sized long wiring.

I'd considered making a leap to lithium iron phosphate (LiFePO4) cells but having seen Jack Rickard nearly burn down his workshop on his electric vehicle web show - EVtv a couple of weeks ago, I was not certain that I'd have the charge control issues all figured out at the first go on big 400Ah cells.  I may do a trial with some much smaller (cheaper if killed) 60Ah lithium cells with my spare Morningstar controller.

In the meantime, I found a good deal on some new 180Ah AGM deep cycle batteries from China by an outfit called Ritar.  These guys usually make VRLA batteries for computer backup supplies (UPS).  But these batteries are usually for "standby" use and not too amenable to being discharged daily.  Although, the Marathon 105Ah packs are also UPS batteries and seem to still be going strong after 1 year and 5 months of daily cycling.
Anyway, this big seller of used UPSs on eBay gets old UPSs and puts new batteries in them (not OEM batteries but these Ritar ones) and then sells the UPSs.  But he had a side line in big deep cycle and EV batteries for toy electric cars and disabled mobility cars and so on.  Ritar RA12-180D packs are slightly different from the RA12-180 packs (which are the UPS "standby" type, whereas the "D" type are supposed to be for "cyclic deep discharge" use).  We'll see...

Very good price:  £178 for 180Ah at the 10 hour rate (most makers quote the more inflated 20 or even 100 hour discharge rates - better makes quote the 10 hour or even 5 hour rate).  Under £1 per Ah.  And that included free next day TNT delivery on a pallet.  I had to order two for my 24V system, and they turned up the next morning even though I ordered them at just after 4pm the previous day.

The batteries needed to be given an initial charge to wake them up from their months of storage in the warehouse and on the ship from China.  Thankfully, AGM batteries do not self-discharge that much (2-3% per month).  Even so, the Ritar technical guide recommended charging them for up to 24 hours before first use.  So I set this up on a mains powered charger and monitored the Voltage and current until neither changed over time.  One battery took the full 24 hours to reach this state; the other was a bit faster at about 18 hours (it read a higher initial Voltage when I first opened the box).
Luckily, it was sunny for a couple of days and so I disconnected the defunct gel batteries and turned off the inverter to let the Marathons come up to something like full charge.

After the new Ritars had both been charged up, I connected them in where the gel battery bank had been and let the packs equalise overnight.  I had to reprogramme the charge controllers a bit as the Ritars like higher Voltages than the gel ones did (although this conflicts a bit with the Marathons that also liked gel Voltages).  I may have to disconnect the Marathons when I run periodic equalisation charges on the Ritars, as the Marathons start to vent gas above 14.40V charge and the Ritars want to go up to 14.60V on an equalisation charge.
Yesterday was one of the windiest days in the UK for over 10 years and gloomy with it, but bright enough to gently charge the new combo and allow some tweaking of the charge controllers.

I mounted the active battery balancer on the distribution board to make things a bit neater.  This can get a bit warm when working hard so I mounted it on stand-off washers; raising the heat sink off the wood.  It will give a bit of extra air flow on the back of the device.

Today was the first use of the whole pack "in anger".  It was wall to wall sunny and we used solar power to heat water (some 2.4kWh went into the water tank).  We even managed to do a load of laundry with solar power again.  The Ritar bank was more highly charged than the Marathons and it did more work initially.  I'll have to see how they balance out over the next few days.

One interesting thing I've noticed is that the very low impedance of the Ritars has really cut down the "twittering" noise the inverter used to make.  I always assumed that it was a "design feature" of the inverter but seems it was a feature of the gel batteries.  Maybe if it starts up again as the Ritars get older, I'll plug my super capacitors into the circuit and see if that helps.

2 comments:

  1. Why don't you just use your capacitors anyway? Would a pair between the solar panels and the battery, along with a pair between the battery and inverter, help to smooth things out and preserve battery-life do you think? They have good prices on 8-50 farad capacitors now due to the big subwoofer craze. I wonder if I should buy before sound-system tastes change? I am excited by the positive effect they had with your small batteries.

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  2. Thanks. I'll have to look into some of these huge new caps that are 50 Farads as that's a lot more energy than the 0.5F cap I have (two 1F caps in series).

    The small cap I have would stop surges on the battery from making my DC lights from flickering but wouldn't do anything to extend the life of the batteries.

    I've seen a guy on Youtube start his car with a bank of ultra capacitors though, so they can deliver huge power. I also saw the same guy run a TV for a few minutes via an inverter from the same caps... With no battery.

    Big caps might upset the solar charge controller that would see the ones on the input as a too high current source when scanning the solar panels for max power point. Might even blow up the controller as the input would massively exceed the 60A current limit.

    Big caps on the output might also be a problem as when first connected they look like a total short circuit. I had problems with the small caps causing the 32A breaker on my small solar demo system to pop as the inrush charge from the battery to the caps was too big. The surge current from those small 7 Ah batteries is only about 100A but the house batteries could probably do 1000A and blow the 100A breakers I have.

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