Mixed Batteries Real Life Issues

System

Hi All,

Long Time Lurker First Time Poster.

I was wondering what the real life implications of a bank of mixed batteries would be / are ?

My situation is we have a Off Grid 48V battery bank made up of 8 AGM 325 AmpHour batteries, 4 of those are doing funny things and not holding a charge and the other 4 seem to be fine. The MFG of the batteries is going to replace them under warranty but the turn around is going to be a couple of weeks we are in rural northern NV.

Rather than run the generator 24/7, a neighbor has some new Trojan T-105 220 AmpHour 6 volt batteries that he will let me have in trade for a little work he needs done on the house.

My question is what would be the "real-world" implications of using this mixed bank of batteries ?

Could I make 2 24 volt banks ? 1 AGM and 1 Wet Cell ? one big string ? like I have now or ???

I have read everywhere is says not to do this but can anyone tell me why ? and what real world issues that a month or two of a mixed bank would have on my solar system / existing AGM batteries ? I would rather just not live on generator 24/7.

Thanks

bmet

Re: Mixed Batteries Real Life Issues

What is your generator's output?

Did your neighbor mention if his new T-105s were having their charge maintained?

System2

Re: Mixed Batteries Real Life Issues

Yes the batteries are less than 60 days from the MFG date of the units and have been floating in his battery shed since he got them.

My Gen is a 12Kw Onan that is propane powered, great generator but expensive to run 24 / 7

BB.

Re: Mixed Batteries Real Life Issues

For a practical point of view--One issue is that (in general) AGM batteries have (effectively) lower internal resistance.

So, at lower current flow, the AGM and Flooded Cell batteries can probably share the pretty well... However, at higher load current, the AGM will tend to output more than its share of current until the AGM's discharge a bit deeper and the output voltage tends to even out the current again.

Similarly as the banks charge, the AGM's may take more of the charging current at higher rates.

Does this mean that a mixed bank will not work--Probably not. More an issue of the batteries may not age well together (AGM's may fail earlier).

(note: I will be talking about 12 volt bank voltages here--just 2x or 4x for 24 or 48 volt battery bank).

The other issue is flooded cell banks should be equalized every month or so (as needed) to 15-15.5 volts or so... And, with AGM batteries, typically high voltage equalization is not recommended (can over heat the catalyst and cause they to vent--leading to a very early death). Although Concorde AGM do seem to tolerate high voltage equalization OK (although, I am not sure this will be good for a long life to do this often--more of a last chance to recover battery capacity before tossing them towards the recycler).

Sulfated Lifeline Concorde AGM Batteries

If I had to mix AGM and Flooded Cell batteries, I would put the AGM on one string and the flooded cell on a second string... And use an A/B/Both switch like this:

Blue Sea Battery Switch 1-2-OFF 350 Amp
Blue Sea 9001e 350 Amp Battery Switch Make-before-break contact design allows switching between battery banks without power interruption Ignition protected - Safe for ... mounting Meets American Boat and Yacht Council (ABYC) requirements for battery switches 3/8"-16 tin-plated copper studs for maximum ...

I would set the charging point for ~14.2 volts with a ~3 hour absorb timer (or whatever your AGM mfg. recommends) monitor the charging current / voltage / battery temperatures and see how everything runs. If the controller has an equalize setting--put that on manual equalize.

If the AGM's seem to perform OK, but the flooded cell are not charging quick enough, I would try ~14.5 volts and see how the batteries perform.

I would monitor bank/string charging current with a clamp on DC amp meter or put current shunts and current meters in each series string.

When you need to equalize the Lead Acid battery string, use the A/B/Both switch to isolate the Flooded Cell String and just equalize that one to 15+ volts when needed (Trojan has a nice battery manual in PDF format).

Trojan Battery 20 page Maintenance FAQ (PDF).
Trojan Battery Maintenance FAQ in español (PDF).

There are just a whole bunch of variables here... Mixing a bank is something that you don't want to do--but if circumstances require it--I don't think it is the end of the world.

With a bit of extra hardware (battery switch, current monitoring by bank, hydrometer for flooded cell battery, isolating the AGM bank so you can measure its resting voltage once in a while to see that it is properly charged and not being over discharged on a regular basis)--You should be able to live OK with the setup (with a bit more care/work to keep things humming along).

And let us know how it works out for you... A few people of done it here before--And, at least in the short term, it did appear to work.

But, I have not done it and I am, in no way, a battery systems expert. Just using my general engineering experiences to try and make something work.

If you cannot get the current between an AGM and the flooded cell banks to be somewhat similar (I would say within a factor of 2x) for discharging/charging (assuming roughly equivalent AH battery string ratings), then I would say that it probably hard to justify a mixed bank. And if there is a 10x or more difference---It does not make any sense at all.

There are things to try (like adding extra cable to the AGM bank to add a little series resistance) and better match the AGM/Flooded Cell current sharing--but that would be a last resort.

If the AH capacity mismatch between the AGM and Flooded Cell strings are too large (possibly over 2x difference, and definitely over 10x difference), I would not recommend mixing the batteries (even if they were all AGM or Flooded cell)--It just is not worth the effort for the extra energy storage.

Battery AH capacity can very by 10-20% between cells/batteries (manufacturing, age, even temperature differences matter). Adding a large capacity string+small capacity string in parallel would hardly make a difference.

-Bill