Setting up my 36v battery system to be charged by solar

jkammer1 · July 2021

Hey there. Picked up a 36v golf cart, (3x12v battery bank) installed two 100w 12v mono solar panels on roof, obtained a 12,24,36,48v 50amp wp5048d solar charge controller to intermediate. It's not seeming to charge at all when configured 12v on panel side, 36v on battery configuration.
My question is; do I need to configure a third panel, run in series, to obtain 36v on both sides for this to work? It seems to work when I break it down 12v -> 12v. But configured 12v -> 36v no dice.
Anyone have any wisdom/experience to lend?

Photowhit · July 2021

jkammer1 said:

Hey there. Picked up a 36v golf cart, (3x12v battery bank) installed two 100w 12v mono solar panels on roof, obtained a 12,24,36,48v 50amp wp5048d solar charge controller to intermediate. It's not seeming to charge at all when configured 12v on panel side, 36v on battery configuration.
My question is; do I need to configure a third panel, run in series, to obtain 36v on both sides for this to work? It seems to work when I break it down 12v -> 12v. But configured 12v -> 36v no dice.
Anyone have any wisdom/experience to lend?

Likely you don't have enough voltage to charge the battery bank. You will need roughly minimum of 45 volts to charge a 36 volt battery bank, perhaps a bit more t equalize it.

Your 12 volt nominal panels likely have a VMP (Voltage under load, like charging) of 17.5-20 volts. So you will need 3 in series.

BTW - That's not a lot of charging for 3 golf cart batteries, 3 in series is likely around 5 amps into a 220 amp battery bank or less than 2.5%. That's more of a maintenance charging than something useful.

MichaelK · July 2021

First, I'd like to ask, why 36V? That is a bit of an oddball these days. Almost all manufacturers have settled on 24 or 48V as system voltages. Outback was the sole company that used to market 36V equipment, and I don't see anything 36V in their current offerings. Do you already have a 36V inverter, or other 36V equipment?

As Photowhit outlines, you don't have enough voltage to charge a 36V battery bank. Assume that bulk charging will start at ~38V, and max out at 44V to reach maximum charge. If your two panels are putting out 18Vmp, then the maximal charging voltage will be ~36V, less than the bulk starting voltage you need. So, as Photowhit indicates, you'll need 3 panels in series to bump up charging voltage to 54V. Then, an MPPT controller will transform the incoming raw solar to exactly the voltage the battery wants.

One further issue is the charging rate. Even with three 12V panels the charging rate is too low. Let's assume that each of your 12V batteries has 120Ah of capacity. You want to charge at up to 1/8th of capacity, so that works out to be 120Ah/8fold = 15A. So, you will need at least 15A of current at least 38V to get good bulk charging.

Since you can expect only to get only about 85% of rated output from each panel on a regular day, you need a 0.85X fudgefactor to compensate for lower panel performance (reciprical of .85 is 1.175X). So, for a 120Ah battery at 36V you need 120Ah X 0.125C X 38Vcharging X 1.175FF = 670W of panels, more than double what is initially suggested.

What I would suggest doing is sell the 12V panels, and replace them with high-voltage grid-tie panels. You can get them dirt-cheap off of Craigslist. I bought 240W panels last year for 55$ each. Get three. Wire them in series to get 90Vmp, and then a MPPT controller will transform the high raw solar voltage to battery voltage, increasing the charging amps as it does that. Alternatively, you could buy four more 12V panels and wire them in a 3S2P configuration. I'll bet though that you'll spend less money buying 3 grid-tie panels then 4 12V panels.

Keep in mind that my numbers are estimates, so if you know what your Ah battery capacity is, plug in the correct numbers. The parameters may change slightly, but the math stays the same.

Setting up my 36v battery system to be charged by solar

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