In my off-grid cabin I currently have a single 258 Ah (C24) Concorde PVX-2580L battery, with a 150 Ah spare (and generator) for backup situations where the main runs flat.

As I frequently see the voltage drop below 12.1V (i.e. the 50% mark) in the winter time, I have bought a pair of 2580's to double my available energy. These are of a different age and series than the one I currently have, and as one is not supposed to mix different AGM batteries on the same circuit, I am now exploring how I can make good use of them.

One thing I just came across is DC-DC chargers, such as this one from Victron. That would allow me for 120$ to charge a battery using the new batteries. That would effectively create a new circuit, so it would easily be under-utilized, if I didn't use that circuit much. I would also need to find space for the behemoth battery the 2580 is :)

Any other input on how I could make good use of the redundant batteries?

  • Before thinking about drawing from two AGM battery banks, how were you planning to get them charged to capacity? – user109695 Dec 11 '19 at 5:16
  • @Jeeped I have solar panels charging them, with a generator + 50A charger for the winter time. – oligofren Dec 11 '19 at 14:44

I would not charge a battery from other batteries; that is insane because it creates conversion losses for no good reason.

However, when one battery is charging, it is fine to tap that energy to charge another battery, using any of a variety of methods. Like any charging, this should be a proper charge controller, not just a diode array.

You don't want to be in a situation of leaving a battery partially discharged for any longer than necessary, because that ages a lead-acid battery.

Lead-acid batteries are a perishable asset

A lot of people, especially who live out in the country and have barn space, don't throw stuff out. They keep it around forever, on the logic that it "saves having to buy it again". Do not do that with lead-acid batteries. A lead-acid battery dumped in the back of the barn for 5 years will be dead as a stone when you go to use it.

Your only options for "surplus" batteries which are still working is a) sell it promptly to someone who can put it to work, or b) put it to work yourself.

So either get rid of it right now... or modify your system to make use of its capacity on a daily basis. Every battery is an addition, because discharge wears lead-acid batteries, and the rate of wear is an exponential function of discharge depth. Draining 2 batteries to 55% full is much worse for total-cost-of-ownership than draining 3 batteries to 70% full.

"Spare for when one runs flat" is also a fail

Similarly, the idea of running one battery pack dead, then switching to the next pack, is a bankrupt idea as far as total cost of ownership. Running a lead-acid pack to failure is absolutely brutal on service life of the battery. A car starting battery is destroyed by 5-10 dips to dead. A battery specifically for solar is destroyed by some dozens of such dips, and worn/damaged/capacity reduced by every single dip.

Again, for best use of the batteries, you want to pull them down as evenly as is practicable. 80-100% won't matter, so pull A to 80%, then B to 80%, then C to 80%, then A to 70%, B to 70%, C to 70%, etc.

Even better, have a smart battery merger/controller that blends all packs together, drawing them down equally; however this requires some intelligence - diodes alone are not enough. (Also diodes take a bare minimum 0.2V drop and most diodes are more like 0.8% -- that's 2-6% of total battery capacity being wasted in heating up diodes.) A manual string of switches (B on, A off) would be more efficient.

Physical topology is also a consideration

If your house is spread out, and 60% of your load is in one place, 25% in another place and 15% in another place... it may be better to distribute your packs around the house so that the batteries are near the loads, rather than "wheeling" (hauling) 12V over long distances. Voltage drop bites 12V very hard: with only 1/10 the voltage of 120V, it has 10x the voltage drop, which means 100x the power drop. You can end up losing a significant fraction of battery capacity to wiring losses, unless you're up on the ought-sizes of wire.

  • Thank you for a full-bodied answer. I am not sure what to do of it, though, meaning you seem to talk about stuff I have no intention of doing or have done (like draining the batteries on a regular basis), and I am left just as clueless as to what to actually do after reading (except the bit about just selling it, which is decent advice). I rent out my cabin and people do dumb things like draining the main battery and then I keep a fully charged spare for those rare cases. At least the Concorde batteries are best-in-class when it comes to handling deep cycling (though not indestructible!). – oligofren Dec 11 '19 at 14:52
  • I am not an electrical engineer, so talking of diodes is a bit above my head. The smart battery merger/controller concept does sound extremely interesting, but I have never heard talk of that in any off-grid forums. Do you have more info on that? – oligofren Dec 11 '19 at 14:55
  • The "smart battery merger/controller" seems like the holy grail for PV home owners. That this doesn't exist makes me think that there's a bit more complexity to it than on first look. I am a programmer, so the feedback logic seems OK to implement, but I have been totally unable to find anything resembling the description. – oligofren Dec 11 '19 at 15:13
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    @oligofren All viable strategies for managing batteries depend on the owner having skin in the game; skill, control and incentive to use the battery responsibly. Completely unrealistic in a rental unit. – Harper - Reinstate Monica Dec 11 '19 at 19:40
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    @oligofren, that's true if they're otherwise exactly the same, but since yours are different brands and ages, they're not. Different brands will sometimes have different proprietary additives to the electrolyte, which can affect the internal resistance and rate of self-discharge, and small differences in age can affect this too. It would work, in the sense that you could run them that way and nothing would blow up, but there's a good chance your weaker batteries will be holding back your stronger ones. – Nate S. Jan 6 '20 at 17:30

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