Current System:
24 volt
2 Multiplus 24 | 3000 | 70 - setup to charge batteries
2 Multiplus II 24 | 3000 | 70 - not setup to charge batteries
2 MPPT 150/70
2 MPPT 150/100
8 Rolls 6v lead acid batteries. S6 L16-HC, 445 Ah. Bank 1 - 4 in series. Bank 2 - 4 in series. Bank 1 and bank 2 in parallel.
It’s cloudy. I am running 2 multiplus 24 | 3000 | 70 from a Generac generator. During absorption they constantly keep the amps at 80+ amps. I have 4 charge controllers. 2 MPPT 150/70 and 2 MPPT 150/100. When it is sunny and they are in absorption, the amps start high. As the resistance builds in the batteries the amps go down to 30.40 + or -. The chargers don’t reduce the amps over time, therefore increasing the temperature of the batteries significantly.
My question is why don’t the Multiplus chargers reduce the amperage over time like the charge controllers do as the resistance builds in the batteries. If there is a setting that does this, please let me know.
You need to find out what the absorption voltage is set at on the Multiplus, it sounds like they are staying in bulk charge as 70A is the rated charge current. See how this compares to the absorption voltage on the MPPTs. Multiplus manuals will have details of reading the settings.
Secondly, do you have temperature sensors and temperature compensation set up. Under high charge rates as lead acid batteries warm up the resistance decreases and the voltage does not increase as they charge so they stay in absorption. This usually occurs with low resistance AGMs though. One way of reducing the risk of this is reducing the absorption voltage, but temperature monitoring and compensation should always be enabled.
Both Multiplus units have the same settings:
Charge Current: 52A
Absorption voltage: 30.50
Float voltage: 27.50
Maximum absorption time: 2h
All 4 of the MPPT charge controllers have the same settings:
Max charge current: 70A
Absorption voltage: 30.20
Float voltage: 27.40
Maximum absorption time: 6h
The system has a SmartBatterySense unit that communicates with the charge controllers via Blue Tooth. The inverter/charges do not have Blue Tooth.
Perhaps the slightly higher absorption voltage on the Multiplus is driving the current into the batteries regardless of the battery state, I would expect it to regulate. Have you checked the actual voltage at the Multiplus terminals during a charge cycle to see how this compares to the 30.5 setting.
Battery makers always seem to suggest a ‘performance’ level as their upper one, so pick the lower. It equates to 29.4V, which is still quite high anyway. Set your mppts to that, and your Multi 0.1V lower. That will give the solar priority when in Absorb.
27V for Float on all chargers (gen should be shut down by then).
Can’t find a Temp Comp factor, but I’d use -4mV/cell/degC, so -48mV (or -27 for F). Don’t forget the ‘minus’ sign.
You can run up to 170A with your 2x strings. That’s 0.2C, which is ok for that type of battery.
A bit of a guess, but when full the A should taper down to ~30A. When that’s reached they’re ready for Float.
