Battery Bank Sizing Questions - Replacing existing batteries.

tr0y · July 2018

Hi All -

So I have a decent sized system in my toy hauler.

The good

• 4 275 Watt PV Panels wired series / parallel - looking at adding one more 300 watt panel because I have the roof space.

• Midnite Solar 150 MPPT Charge Controller - #4 wire to batteries

• 2000 Watt Pure Sine Wave Inverter Charger - 1/0 wire to batteries

The Bad
• 3 155 Ah / 12v Interstate telecom batteries that are dying / dead ( yes I knew that they would do this but they were $40.00 for all 3 )
• 1/0 battery cables.

Out of the panels I have seen up to about 900 Watts

Out of the CC is anywhere between 30 and 60 Amps

As was expected the Telco batteries have died and as much as I would like to do something exotic I have access to some 200 Ah 12v AGM true deep cycle batteries for a reasonable price. These batteries were manufactured 03/2018 and been in float service since with 8 to 10 discharges to 75%

Can anyone point me to the correct calculations on how many I should install ? I have the space and weight carrying capability to install up to six (6) of these in the toy hauler. I just don't want to install to many and not have the ability to charge properly.

Thanks !

Estragon · July 2018

It depends on your use, and whether you have alternate means of charging (generator, shore power) and are willing to use it.

For occasional (eg weekend) use, something like 5% of 20hr capacity would be about minimum. For example, with 30a output from controller, 30 ÷ .05 = 600ah max battery. Bank may not both support loads and fully charge on the weekend, but has the rest of the week to catch up. For more full time use, a 10% number would be more appropriate, so 30a ÷ .1 = 300ah max. Depending on location and climate, you may still need additional charging capacity. These numbers assume daily loads of up to ~25% of capacity.

If you can charge with a generator, you can get away with a bigger bank by doing some of tge bulk charge with the generator, and letting solar finish.

tr0y · July 2018

Thanks @Estragon I go in spurts on weekending and full timing. Come January I am going to do 4 to 6 months of FT while I build a house. As for location most of the time I am in the western US.

I do have access to both an in-frame 4000 watt generator and a 2000 watt Honda. So it would seem that adding another panel or 2 if I can find the space. and having the ability to access generator or shore power might get me into the ability to put 2 and maybe 3 of the 200 Ah batteries into the RV ??? Does that seem to far off ??

Thanks

Estragon · July 2018

Jan/Feb might be tough, but as long as you can/will use the genny it seems okay to me to use 2 or 3. If using 3, each should be fused individually.

tr0y · July 2018

Thanks for the help ! Economics just made my determination. I picked up one more panel to match the rest for $100.00 brand new and there is no more roof space now and at the same time I got three nearly new ( MFG date 03/18) 12v / 210Ah AGM batteries at a great price from a crashed bus. So it looks like a 600Ah bank and 1375 watts of PV is what I get. I don't think this should be bad for a travel trailer, I am pretty certain that many people do fine with less.

BB. · July 2018

Review this website for how to series/parallel batteries to have "balanced" current flow:

http://www.smartgauge.co.uk/batt_con.html

-Bill

littleharbor2 · July 2018

Here's another page from their site that will take you to lots of good info.

http://www.smartgauge.co.uk/technical1.html#combi

Battery Bank Sizing Questions - Replacing existing batteries.

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