Battery Bank Sizing Questions - Replacing existing batteries.

tr0y

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

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

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

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

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.

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

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

-Bill

littleharbor2

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

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