How to run a 75ah and 100ah battery at the same time

[Jeff Wright]

Hi guys and gals, I've been running a 75 ah AGM battery in my ATC bobcat. I've got an isolator and 100 watt solar panel. Normally I have enough juice for what I need but lately I've been doing more winter camping and the fan from the furnace running much of the night seems to suck more power than I'm used to (also lower and less sun in the winter is a factor as well).

I was contemplating buying another 75ah battery when a friend just gave me a year old 100wh AGM. I've heard running two different capacities in parallel is bad for at least one of the batteries. I'm sure I could get one of these selector switches to select which one is being charged/discharged:

https://www.amazon.com/Attwood-4-Way-Battery-Selector-Switch/dp/B0000AZ7X6/ref=sr_1_9?ie=UTF8&qid=1515089459&sr=8-9&keywords=battery+selector+switch

But I'd really like to run them in parallel and just not worry about switching back and forth to charge and discharge one at a time. How much damage would I do?

Thanks!

 

[ardvark]

You will likely shorten the life of one battery. How much depends on multiple variables. I don't think there is any way to answer this question directly without putting a meter on the batteries and check voltages. I am guessing in addition to being different sizes, the batteries may also be different ages. My two cents.

 

[rando]

Put them in parallel and don't worry about it. Yes it is not ideal and will likely lead to slightly less than optimal lifetime, but it will be better than running your current battery way down on a regular basis. If you have close to enough capacity on the 75Ah battery , you could also just switch it out for the 100Ah battery and buy an extra 30%.

 

[Casa Escarlata Robles Too]

First no expert here,I also have a Bobcat with a 75a battery,but when I bought a 100a battery for our 2016 trip to Alaska I was tempted to go even larger but the recommendation was that with my solar 105w a larger amp then 100a battery wouldn't charge up fully.This is from the battery co.
So I just went with the 100a. I don't think that using both battery's together there isn't enough output from the solar to completely charge them both. The battery I have is a V Max brand. You can search their site for the charts that give size info for the amount of solar needed.

As Rando said just use one battery at a time.It would be PITA to change them back and forth but might be the best solution.
Frank

 

[Vic Harder]

the recommendation was that with my solar 105w a larger amp then 100a battery wouldn't charge up fully.This is from the battery co.
So I just went with the 100a. I don't think that using both battery's together there isn't enough output from the solar to completely charge them both.

Frank, that is a good rule of thumb, but... it depends on if you are planning to discharge it fully every night and if you are ONLY charging via solar. The battery co may be assuming a home/industrial solar install where optimum design is that they DO fully drain the batteries every night, in order to maximize their investment.

I'm no expert either, but I don't think this is a rule to be applied to campers without getting the full picture.

 

[Casa Escarlata Robles Too]

Frank, that is a good rule of thumb, but... it depends on if you are planning to discharge it fully every night and if you are ONLY charging via solar. The battery co may be assuming a home/industrial solar install where optimum design is that they DO fully drain the batteries every night, in order to maximize their investment.

I'm no expert either, but I don't think this is a rule to be applied to campers without getting the full picture.

Thanks Vic.Although the battery and the chart used were for RV/solar usage.
But I would still go with a larger solar out put to charge that battery set up ,150a together.
Frank

 

[Vic Harder]

But I would still go with a larger solar out put to charge that battery set up ,150a together.
Frank

Me too. ;-)

 

[Jeff Wright]

Hi Everybody, Thanks for the advice, seeming like it's not going to ruin anything by hooking them both up. My brother in law is in the solar business and offered me a free 150 watt panel so I'll either swap it out or add it to my setup. He's in Oregon and I'm in California so it might be a while before I'm up there next. I think I might buy one of the original selector switches https://www.amazon.c...selector switch that I was talking about. If I was having problems getting them topped off I could charge them individually. I'll also be mostly relying on my isolator in the winter as the sun isn't the effective at charging anyways. Will be nice to be able to go a couple of days without worrying.

 

[Casa Escarlata Robles Too]

Jeff you said your BIL is in the solar business. Is that house solar.
Can you use panels designed for homes on a 12v RV system?
I just had added to my house solar and noticed the panels don't have an out put of 12v but a higher voltage.
You might want to double check the voltage.
12v panels are really inexpensive I would hate to ruin a 12v system with a wrong panel.
Just my thoughts.
Frank

PS I just looked at the info on my house panels.They are Sunpower and have a 360w/240v out put and if I read the specs right they are AC power.

 

[Vic Harder]

Jeff you said your BIL is in the solar business. Is that house solar.
Can you use panels designed for homes on a 12v RV system?
I just had added to my house solar and noticed the panels don't have an out put of 12v but a higher voltage.
You might want to double check the voltage.
12v panels are really inexpensive I would hate to ruin a 12v system with a wrong panel.
Just my thoughts.
Frank

PS I just looked at the info on my house panels.They are Sunpower and have a 360w/240v out put and if I read the specs right they are AC power.

Yes, you can... those 265W I have on mine are for home solar installs. You just need the right solar controller. And, I wonder if those Sunpower panels you have are not 24V... not 240?

 

[Jeff Wright]

Jeff you said your BIL is in the solar business. Is that house solar.
Can you use panels designed for homes on a 12v RV system?
I just had added to my house solar and noticed the panels don't have an out put of 12v but a higher voltage.
You might want to double check the voltage.
12v panels are really inexpensive I would hate to ruin a 12v system with a wrong panel.
Just my thoughts.
Frank

PS I just looked at the info on my house panels.They are Sunpower and have a 360w/240v out put and if I read the specs right they are AC power.

Thanks Casa! He is in house solar but I think many panels are essentially the same thing as a 12 volt RV panel and feed a different kind of controller that converts them to 120. I'll double check with him but he definitely knows my use case and knows more than anyone I know about solar and off grid 12 volt systems as well.

 

[Casa Escarlata Robles Too]

Yes, you can... those 265W I have on mine are for home solar installs. You just need the right solar controller. And, I wonder if those Sunpower panels you have are not 24V... not 240?

Thanks Vic.I double checked and the speck sheet reads 240v. When I see the solar guy Monday I'll have to ask about that.
Frank

 

[klahanie]

Jeff, I want to post the link below for two reasons,

First, don't know about the amazon one but, just so you are aware, some switches can be surface mounted, if that makes it easier/better for you.

Second, don't know your plan but - and I could be way off - I think this switch might work to isolate or combine for both charging and discharging. All in one. see documentation tab, instructions, last page for wiring.

I'll add, it's a good site to look around for quality stuff ...


https://www.defender.com/product3.jsp?name=blue-sea-systems-m-series-mini-dual-circuit-plus-battery-switch&path=-1|328|2290051|2290060&id=838766

 

[Casa Escarlata Robles Too]

Yes, you can... those 265W I have on mine are for home solar installs. You just need the right solar controller. And, I wonder if those Sunpower panels you have are not 24V... not 240?

Vic,I just talked to the person from the solar company doing the install of my house system.
The Sunpower panels are 240v AC panels.He said newer panels for home use have a micro inverter in them and they produce 240v AC right from the panel.So could you still use them and what type solar controller would you need for the camper?
Also each cell is designed so that a shadow over any part of the panel won't stop the power out put for the whole panel.
Frank

 

[Vic Harder]

cool panels! No, I don't think you can use those on your camper, as I am not aware of a controller that would work. Very efficient design for a house though!

 

[Casa Escarlata Robles Too]

cool panels! No, I don't think you can use those on your camper, as I am not aware of a controller that would work. Very efficient design for a house though!

Thanks Vic.Not thinking about using them,I have enough with my system,but with the size and output/weight
they would be nice.
Frank