Using old house batteries to charge new batteries?

[bherdrick]

I have a Thor Tellaro 21J that came with 2 100amp lithium house batteries. I upgraded to larger capacity batteries, but now have the old ones just laying around. I have been trying to sell them to no avail, but then had what seemed like a good idea: buy an additional inverter and bring the old batteries with me on the road. When the installed house batteries get low, plug in the old batteries into my RV (with an adapter) to the inverter to charge the new batteries up a bit. Here is what I was trying (the battery isn't connected to the inverter in the photo):



I didn't seem to get a charge at all and the control panel was giving me an error. Is the inverter not the right kind or maybe the whole concept is flawed?

 

[DavidEM]

Lots of efficiency losses in your scheme, but it should work. If you can predict that the house batteries will get low, it will be cheaper and much more efficient to wire the old batteries in parallel from the get go. Don’t know what would happen if you wired them in parallel after they got low. Suspect you would see a high current between the two types, one trying to charge the other and it might trip the BM.

David

 

[dkoldman]

Buy something like this and call it a day. I carry one in all Cars\RV at all times.

https://www.amazon.com/AVAPOW-Start...6&psc=1&mcid=6ad3c658700d317a8f8ae6692e655c03

 

[bherdrick]

Well, my whole goal was to get some use out of my extra 2 x 100amp batteries. I don't think that item would do that.

 

[Chateau_Nomad]

Charge ALL batteries to 100%... when the charger stops. Then (as David said) wire all the batteries in parallel to create one large bank.

Issues: Having mismatched battery capacities may prevent one or the other from completely recharging, but they will all eventually discharge... using the full capacity.

Also, depending on how many cycles the older batteries have gone through, they may add to the "fully rechargeable" issue.

Regardless, I'd try this before selling... close to 200Ah is quite a bonus for a backup if all works!

 

[bherdrick]

The new batteries say that they shouldn't be connected in parallel with any ones that are different amperage or different brand.

"Products of different manufacturers cannot be connected in parallel.When the batteries are used in parallel, the voltage must be the same."

https://www.temgot.com/products/tem...ttery-perfect-for-rvs-motorhomes-and-off-grid

 

[Mark54]

The new batteries say that they shouldn't be connected in parallel with any ones that are different amperage or different brand.

"Products of different manufacturers cannot be connected in parallel.When the batteries are used in parallel, the voltage must be the same."

https://www.temgot.com/products/tem...ttery-perfect-for-rvs-motorhomes-and-off-grid

If I were in your position, I would make a 2nd bank, and get an "AB" switch to go from one bank to the other- they would be separate, but would use the same charging systems, and give you 2400Wh of additional power.

If you join the two sets, they should both be the same type of battery- LiFePO4 with same type and voltage. Amps isn't the issue as much as type and voltage.


You can get solar charge controllers with dual battery banks too.

This device below can be your AB switch. When the big batteries get low, you just switch over to smaller set.

 

[bherdrick]

I agree, Mark, that would be ideal. As you are aware, I am not skilled at modifying my RV myself. Getting these new batteries in was a month long project with many trials and tribulations. I am wary to try another project similar to that one in any way. I really wanted something quick and dirty that would only get used infrequently.

 

[bherdrick]

It definitely isn't working with the janky inverter setup that I have tried.

 

[16ACE27]

Charge ALL batteries to 100%... when the charger stops. Then (as David said) wire all the batteries in parallel to create one large bank.

Issues: Having mismatched battery capacities may prevent one or the other from completely recharging, but they will all eventually discharge... using the full capacity.

Also, depending on how many cycles the older batteries have gone through, they may add to the "fully rechargeable" issue.

Regardless, I'd try this before selling... close to 200Ah is quite a bonus for a backup if all works!

The new batteries say that they shouldn't be connected in parallel with any ones that are different amperage or different brand.

"Products of different manufacturers cannot be connected in parallel.When the batteries are used in parallel, the voltage must be the same."

12V 460Ah Lithium Battery, Perfect for RVs, Motorhomes, and Off Grid

That outdated parallel thinking does not apply to LiFePO4 batteries of the same nominal voltage which each contain their own BMS.

Install the old batteries in their own enclosure with the combined ground attached to the chassis and the combined positive going through an appropriate sized breaker, cable, and disconnect switch. Decide when you want to parallel for added capacity and for charging.

 

[Chateau_Nomad]

That outdated parallel thinking does not apply to LiFePO4 batteries of the same nominal voltage which each contain their own BMS.

Install the old batteries in their own enclosure with the combined ground attached to the chassis and the combined positive going through an appropriate sized breaker, cable, and disconnect switch. Decide when you want to parallel for added capacity and for charging.

I gotta disagree with you on this one ACE. What I described doesn't apply to FLA, but specifically to LiFePO4... but ONLY in parallel. The BMS differential would likely only apply with high amperage draw/discharge - obviously tripping the lower rated BMS. Or you could also fuse for the smaller battery rating too... but I'd still cable for the larger battery.

As long as you're not pulling massive amp loads, like an air conditioner I don't see an issue... the video explains that. He got the full rated capacity from VERY dissimilar amp rated LiFePO4 batteries.

So... why add complexity by putting the batteries in a separate bank and separate circuit? Simply chain them in PARALLEL and be done. The video demonstration shows precisely how well this worked.

 

[dkoldman]

Well, my whole goal was to get some use out of my extra 2 x 100amp batteries. I don't think that item would do that.

You are correct, I misread what you were trying to do. I thought were talking about a backup to start if batteries were too low.

I have the rest of your quest and ideas. You are on you own

 

[FLYINGPIG]

Hi,
Did you replace the same location above the axle?
What is the capacity of new B?
Can you post some pictures?

Thanks,
Chris

 

[bherdrick]

Yes, I replaced it in the exact same position. It was a pain in the ass. I had to get new boxes fabricated since the new batteries are larger. They were able to keep the existing holes so I could attach them to the existing bolts. With the larger box size, it was difficult getting them into the slot that the smaller boxes were in. I used a tire jack to raise the new battery inside the new box up to the bolts. The new batteries are 460Ah each. I don't really have any photos of it.

 

[FLYINGPIG]

Thanks, Bherdrick. Great job!
After 2 years, my batteries are only good for the refrigerator and lights. During my last trip to Canada at around 30K miles, the 2nd alternator got burned up and suck the juice out of the batteries in 10 minutes. For a temporary measure, I disconnected the alternator and installed a small LiFepo4 charger directly on the battery. This setup is great while parked in storage. If I turn off the B switch, it takes up 3 % a day (under covered storage-no solar) and about 40% with B switch on. I always thought about going with a bigger B bank but not sure worth it. I think Mark54 did a great job and don't like the idea of keeping the battery outside. There was a s* load of wires under the belly and I was scared. Haha.
Thanks for sharing yours.
Chris