*Alt Power Battery Bank*
Mixing Different Size And Age Batteries
By: Spitfire
28 March 2011

Conventional wisdom and good engineering practices have always said that batteries in a bank should be the same ampacity (size) and approximately the same age. This is still good advice if you can afford to do it but, if you’re like us, we only have a budget to build our system a little at a time.

We have a 24VDC inverter system powered initially by two strings of four 6VDC, L16 type batteries. Calculations showed we'd need 3 strings, but we couldn't afford it at the time. A full year later, our budget improved, and we got the 3rd string. No problems were detected with the different age batteries. Three years in, one of the original L16's developed a leak in the bottom of the case, so we replaced it with a new L16; no problems detected.

We added the kitchen fridge to the system, requiring more batteries. The L16's were too much for the budget, so we got the equivalent power with 8 each Walmart 27DC6 batteries, wired as 4 strings of 2 batteries each. That was a year ago and, so far, no problems detected.

This flies in the face of conventional wisdom. Not only did we put different age batteries in the same bank, but we used different manufacturers and different ampacity batteries. Not only do the batteries appear to be holding up fine, but numerous measurements with a DC ammeter show both charging and discharging within a few percent of each other. Likewise, hydrometer readings are extremely close to each other.

There are some conditions that I believe have allowed us to do this successfully:

  1. All the batteries are the same chemistry type.
  2. All the cables connecting the batteries are big enough to reduce any unequal charging.
  3. The battery bank is equalized every month.
  4. Water levels are topped up every month.
  5. We use a Battery Minder desulphator.
  6. The batteries live in a nearly constant 60 degrees Fahrenheit environment.
  7. The batteries are charged by high end Outback inverters and a high end MPPT Outback charge controller.

So, “everyone knows” you shouldn't do it, but we did. And, after several years, its working fine. YMMV

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