Mixing AGM batteries from different manufacturers?

[ADKGuy]

I have a Renogy 100 Watt suitcase and 100 Ah AGM battery on the way. If I want to add another battery down the line does it have to be a Renogy battery?

If mixing manufacturers is ok, what about sizes? I have a 45 Ah AGM Thunderbolt Magnum battery from Harbor Freight. Can I wire that in with the Renogy?

Thanks, this is all new to me and I’m trying ingest the over abundance of information!

 

[GXMnow]

Batteries of the same chemistry and cell count can usually be run in parallel without a problem. The only big pain is they need to be brought to the same level of charge before you connect them together. And they should be separately fused. You also want to try and keep the resistances close. Same length and gauge of cables. If you maximum current is less than the single largest battery can handle, you should be fine adding a smaller battery to extend run time, but do not expect to get any more current. If you do add a smaller capacity battery, it may be a good idea to intentionally wire it with a slightly smaller cable. This way, the bulk of the current will come from the larger battery, and the small one will just help charge the larger one. Even if you add a battery of the same ratings, you will only likely get about 50% more current. That allows for some imbalance without blowing fuses.

 

[ADKGuy]

Thanks for the reply. How do I know how many cells a battery has? The “instruction” sheet that came with the Thunderbolt is less than helpful. ?

For now it’s a basic 12 volt system. I just want to run my cpap machine with the 12 volt adapter overnight while camping.

 

[GXMnow]

12 volt AGM should be 6 cells. The cells run about 2.2 volts average, or 13.2 volts for 6 cells. Fully charged will hit about 14 volts. You should not run them below 50% charge as it will shorten their life. 50% charge is still about 12.75 volts or 2.125 per cell in a normal 6 cell "12 volt" battery. Do you know how much power your CPAP requires? Either watts or amps at 12 volts will work to calculate it out.

The Renogy 100 watt suitcase kit is not a lot of power, and the charge controller it comes with is not very efficient. If it is in good sun all day, you might get 400 watt hours or so of power to charge the battery. As long as you use less power than the solar panel can make you will be fine, but you should check the numbers to see. More battery will only help so much if you are working at a loss from the solar input.

 

[ADKGuy]

Best I can figure it he cpap is 90 Watts but I don’t think it runs 90 Watts all the time and it varies on the various settings and since it auto adjusts as you sleep I imagine it fluctuates then.
I don’t know how to measure the 12 volt pull it has. I’ve seen people say to use the 12 volt adapter. I’m guessing because it’s more efficient. So does measuring the 110 draw with a Kill-A-Watt give me any info. Then of course it won’t know it’s true draw since I’ll be asleep.

 

[GXMnow]

I have one of the cheap Kill-A-Watt units from Harbor Freight and it does store the total watt hours used. Reset it, and run your CPAP through it for a few days and you will get a total watt hours used. Divide by the number of days and you will get an average usage per day. If it is less than 400 watt hours a day, then that 100 watt suitcase solar panel might be able to keep up.

The 100 amp hour 12 volt battery, being AGM lead acid, you can use about 50 amp hours without hurting it. So 50 amp hours x 12 volts = 600 watt hours. If the battery was fully charged during the day, that would provide 90 watts for about 6 hours and 40 minutes. I assume the sun won't be shining while you are sleeping. So hopefully the average draw for a full night is a bit less than 90 watts.

 

[Luthj]

I don't recommend mixing different ages of lead batteries if it can be avoided.

It can perform acceptably if the following are true

-Current less than C/4 the vast majority of the time. Ideally C/10.
-Good charging is available. (Pack doesn't sit a lot at partial SOC).

That being said if you don't mind a bit shorter life, mixing batteries of the same voltage, but different MFGs/capacities does function.


Since lead batteries have significant internal resistance, as long as they are within 0.25V, you can connect them up without worrying about cross charging.

 

[ADKGuy]

I have one of the cheap Kill-A-Watt units from Harbor Freight and it does store the total watt hours used. Reset it, and run your CPAP through it for a few days and you will get a total watt hours used. Divide by the number of days and you will get an average usage per day. If it is less than 400 watt hours a day, then that 100 watt suitcase solar panel might be able to keep up.

The 100 amp hour 12 volt battery, being AGM lead acid, you can use about 50 amp hours without hurting it. So 50 amp hours x 12 volts = 600 watt hours. If the battery was fully charged during the day, that would provide 90 watts for about 6 hours and 40 minutes. I assume the sun won't be shining while you are sleeping. So hopefully the average draw for a full night is a bit less than 90 watts.

Thanks this is helpful. I’ll pick one of those up.

 

[ADKGuy]

I don't recommend mixing different ages of lead batteries if it can be avoided.

It can perform acceptably if the following are true

-Current less than C/4 the vast majority of the time. Ideally C/10.
-Good charging is available. (Pack doesn't sit a lot at partial SOC).

That being said if you don't mind a bit shorter life, mixing batteries of the same voltage, but different MFGs/capacities does function.


Since lead batteries have significant internal resistance, as long as they are within 0.25V, you can connect them up without worrying about cross charging.

So that kind of nixes my plan to add another 100 Watt battery later. As of right now these won’t be used very often only when I go camping. I can keep it hooked to a trickle charger in the off times if that helps.

Eventually the plan is to have a camper van with 400 Watts of solar on the roof and using the suitcase to supplement that.

 

[GXMnow]

What he said is pretty similar to what I said. When you parallel different batteries of the same voltage, you can get more run time, but you can't expect them to give more peak power. The power used should be low to allow the batteries to share the load. C/4 is 1/4 of the amp hour rating. So a 100 amp hour battery running at under 25 amps. 25 amps at 12 volts is 300 watts. C/10 is 10 amps at 12 volts = 120 watts. This is still more than the 90 watts you are talking about.

As batteries age, their voltage curve can change a bit. It is best to use the same age of battery, but as long as the old one is still holding a good charge, and not self discharging as it sits, it should still add capacity to the system. Since we are talking only putting batteries in parallel, it is not as big of a deal. The stronger battery will naturally pull more of the load. As long as the weakest battery can run the load, they will all help out.

I think you bigger issue here is going to be getting enough charge from the solar panel.

 

[Luthj]

100W isn't a lot of charging. You can work out the harvest for your region using online calculators.

 

[ADKGuy]

How about this? I also have the 100 watt Harbor Freight solar panel kit. I can’t tell if they’re wired in series or parallel because the panels don’t connect to each other. They connect individually to a hub which then goes into the charge controller.
From what I’m seeing the connector is called an SAE connector.
I found an SAE to MC4 connector adapter on Amazon.
Can I use that with an MC4 combiner to hook my suitcase and the Harbor Freight panels to the Voyager charge controller on my suitcase?