Electrical Myths, Part 3: Mixing Batteries of Different Ages/Capacities

We’ve all been there at some point–a battery in a multi-battery bank is in need of replacement, and the salesperson is telling us that they should all be replaced together.  Is mixing batteries really a bad idea? Or is it just a ploy to sell more batteries?

Well, for the most part it’s salesmanship.  Let’s think about what’s going on when we charge batteries in parallel.  First of all, assuming solid connections, the voltage across each battery will always be the same.  What does that mean?

How Battery Charging Works with a Parallel Battery Bank

Let’s suppose you have 3 different 12V batteries, wired in parallel to supply 12V power to your RV.  They can have different capacities on account of size or age, but the same chemistry (e.g. all flooded lead acid or all AGM).  Before you start charging, the voltage across each of them is the same–even if one is fully charged and the others aren’t.  Charge will flow from one battery to the other two until they’re balanced.  With a lead acid battery bank, the internal resistances are limiting to a point that you don’t have to worry about arcing or your battery cables overheating when you connect them (not the case with lithium-ion banks…).

So when we start charging, all of the battery banks are very close to the same point as far as state of charge.  Each battery acts like a resistive load, and current will flow to the battery with the lowest resistance, or highest capacity, more than the rest.  All of them will reach the end of bulk charging together, when they’ve all reached the same voltage (remember, they’re connected together–they have no choice!).

Same thing through the absorption phase–current flowing to each battery is proportional to its capacity, as they rise in voltage together.  Transitioning to the float stage, all will be held fully charged, again at the same voltage.

So can I do it with Batteries Wired in Parallel?

There’s absolutely no reason for an aged battery to be excluded from a battery for electrical reasons–only for space and weight would that make sense.  Would a new battery have greater capacity than an old one? Sure.  But the one you have is free while a new one isn’t.  If you need or want the extra capacity, go for it–either for longer runtime with house loads, or more cranking power. But your old battery isn’t going to ruin the new ones.

Mixing Batteries in Series

It’s common in many RVs to make use of pairs of 6V deep cycle batteries wired in series.  In a pair of 6V batteries in series, the voltages of each are not guaranteed to be the same as they are when wired in parallel.

What this means is that as the batteries discharge, voltage on the one with lower capacity will fall faster.  You’ll take it out of a safe (for longevity) state of charge range, without the voltage across the series batteries indicating that.  When you charge the batteries, the reverse happens–the battery with lower capacity will be charged faster, and can be overcharged.

You can probably see where this is going–the battery with lower capacity degrades faster, lowering its capacity, which accelerates the process.

Doesn’t an Equalization Charge Fix That?

Not quite.  An equalization charge will get cells balanced at the fully-charged end of the scale.  But it doesn’t keep them there as you discharge two different batteries in series.  You also don’t want to do an equalization charge frequently.  When you do, you’re overcharging the batteries to get them in balance so that a single cell doesn’t get discharged too far (good thing), but that overcharging does shorten the life of each cell (bad thing).

What about Batteries with Shorted Cells?

Ok, now we’re talking about something a little different.  In the case of a battery shorted cell, you’ll be overcharging the other 5 cells in the same battery.  They’ll continue to draw power from the others in the bank, and that’s a battery that should be taken out–it’s basically not going to contribute any energy until you’re discharged  below where the other batteries should be operating.

How do I test battery capacity?

One of the easiest ways to test individual battery capacity is with a portable analyzer.  If you only have 12V batteries, they’re pretty inexpensive, like this one for $42.74.

If you have 6V batteries though, you’ll probably want one capable of analyzing an individual 6V batteries.  You can make do with a 12V tester, but to match batteries you’ll have to connect various combinations in pairs to figure them out individually–which is time consuming.  A good 6 and 12V tester can be found here, but costs a little more, at $108.99.




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