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Hey All.....

I have 2 Battery banks hooked in parallel on my 24v system:

1. (4) 6V Trojan L16's.
2. (12) 2V Trojan L16's.

Both banks connect to a common system buss bar from which the inverter gets it's power, and also the charge controller inputs it's power.

Both banks have equal length 2/0 cables running to them, about 10ft long.

Question......how bad is this design.......lol....... and what are the consequences of running my system like this.

Thanks, z

Only batteries from the same lot/batch can be considered equal.

When un-equal batteries are paired, the worst/lowest battery always drags the best battery down to the low level, and this process of walking the best battery down,  can take weeks or months, and then the battery bank won't hold a charge overnight. 

More than two parallel batteries becomes a candidate for imbalance and it's just a game till something dies

here's a page with a good explanation
http://www.smartgauge.co.uk/batt_con.html

Quote from: mike90045 on April 01, 2019, 11:33:41 PM
Only batteries from the same lot/batch can be considered equal.

When un-equal batteries are paired, the worst/lowest battery always drags the best battery down to the low level, and this process of walking the best battery down,  can take weeks or months, and then the battery bank won't hold a charge overnight. 

More than two parallel batteries becomes a candidate for imbalance and it's just a game till something dies

here's a page with a good explanation
http://www.smartgauge.co.uk/batt_con.html

Thanks Mike......I hooked some mini voltage meters to the 6V ones, and every 3 of the 2 volt ones to get a comparison.

Right now the 6V ones are at 7.07, 7.08, 7.12, 7.13
The 2V groups are at 7.23, 7.20, 7.18, 7.22

System Voltage is at 28.5 (Absorb)

I guess I'm gonna find out what happens huh?....lol

Thanks for that link..........cool stuff........

But both banks are hooked in series (each one at 24v), and there's no way to cross link them cause they're about 10' apart.....

z

Ifs and Buts:

If you had a diode at the weaker bank's output terminals to the power bus then the weakest bank would only kick in when needed... if those diodes could handle the current. If that diode were made of switching MOSFETs the voltage drop could be reduced to a smart 0.1 Volts.

More diode would be needed on the PV input side as well if they are both charged from the same array. This gets complicated and as far as I know there is no commercial solution and there is a fair amount of design to do, but for the fiscally poor and the environmentally strong among us who are already reusing old batteries this is probably the only way to do it well. Failure modes are bound to be problematic with MOSFETs failing shorted, but that could be detected at least.