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scott-laird asked

Slow charging/high voltage drop with SmartBMS CL 12/100 in camper

I'm trying to debug a slow charging problem. I have a SmartBMS CL 12/100 set for 90A, but I rarely see over 50A, even though my battery, alternator, and wiring should be more than capable of faster charging. If my Lithium battery is under 5% charged, then I *might* see a brief spike to 80A for a minute or two when I start my engine, after that it drops back to 50A within a few minutes. By the time the battery is half full, I'm lucky to see 30A. It takes 5 or 6 hours to fully charge the 200Ah LFP battery off of the alternator.

I'm seeing 13.8V on the input side of the SmartBMS with the engine idling, but voltage on the output side is down to 13.4V, as seen by the SmartBMS itself, multiple SmartShunts, and a multimeter across the SmartBMS. Voltage is more or less constant (13.4V relative to ground) through the whole DC system downstream of the SmartBMS.


I have a SmartBMS CL 12/100 connected like this:

Alternator (280A, +100A over stock, not smart) -> vehicle battery -> 100A fuse -> 15' AWG 2 -> Disconnect Switch -> SmartBMS

The output of the BMS is SmartBMS -> 500A SmartShunt -> Disconnect Switch -> 125A fuse -> bus bar -> 125A fuse -> 500A SmartShunt -> 200Ah Victron LFP battery.

All of the relevant wiring here is AWG 2 (35 mm^2). There are additional devices connected to the bus bar, but they only total ~2A at the moment.


All connectors appear to be tight. I generally only have 1 or 2 wires connected to each terminal, stacked in the correct order, with washers on top, using marine-grade fittings and heat shrink. Nothing appears to be overheating in any way. It's just charging slowly. Nothing has changed in ~9 months.

As a controlled test, I tried bypassing the SmartBMS briefly. Without the BMS, I saw roughly twice the charging rate, although the battery was nearly full at the time. The voltage at the battery jumped up to roughly match the BMS input-side voltage.

I *think* I'm not getting enough voltage to the battery through the SmartBMS to charge at a reasonable rate. As the battery voltage goes up, the voltage difference between the charger and the battery drops, so the charging rate falls..

Is a a 0.4V drop across a SmartBMS CL normal? Does anyone have anything else that I should try, other than replacing the SmartBMS with a giant DC-DC charger?



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