Can I Connect 37V and 12V Batteries?

[diy50per]

I have a question that may seem odd, but I am really searching for an answer.

I have several 37 volt lithium ion bicycle batteries that hold a lot
of charge after being recharged through grid outlets.

I also have an old RV that uses 12 volt appliances run from 12 volt
lead acid batteries, and solar panels on the RV roof, feeding the 12
volt batteries.

Now I wonder whether the following idea makes sense: to connect two
empty 12 volt batteries in series so they be a 24 volt bank, and then
connect one 37 volt lithium battery in parallel.

The idea would be to drain the charge of the 37 volt lithium battery
"downhill" causing the empty 24 volt system to become charged,
hopefully without much wasting of energy.

 

[sophiecentaur]

Steer clear of using L-Ion batteries in conjunction with any devices not specifically designed for them. They are very useful beasts but need to be treated with respect or they can explode if not charged and discharged within a specified range of conditions. Seriously, if you need to be asking these questions then I think you would need to learn a lot more before trying the scheme out that you propose.
All the literature spells out the risks very clearly.

 

[diy50per]

Thanks a lot, indeed. Yes, I have a lot to learn. Can you point out
some literature on the subject? Guess I will be stuck with lead-acid
batteries, then. What would you suggest for an RV with solar panels?
Car batteries don't take the deep cycles, and deep-cycle batteries
are, I guess, not suited for the constant fluctuations. Plus they are
too heavy to handle and maintain alone. Currently I am trying out
multiple motorcycle-size batteries, but they aren't genuine motorcycle
batteries. They are sealed, valve-regulated and rated in amh, not
cold-cranking specs.

 

[sophiecentaur]

Sorry - I have never been involved with batteries at a technical level - except for Lead Acids in cars and on boats. I have just read so many comments about the extra level of competence needed to engineer with Li Ions that I have not bothered to get involved.
Of course, they have an awful lot going for them, if you can acquire the knowledge and keep your eyebrows intact.

 

[Windadct]

I have to agree about the risk...If a 1000 man team at Boeing can't figure out how to manage these on their "vehicle"...

 

[sophiecentaur]

I have to agree about the risk...If a 1000 man team at Boeing can't figure out how to manage these on their "vehicle"...

Cruel. Anyone can make a simple mistake!

 

[jfgobin]

Now I wonder whether the following idea makes sense: to connect two empty 12 volt batteries in series so they be a 24 volt bank, and then
connect one 37 volt lithium battery in parallel.

You will create a short circuit of the Li battery through the 2 Lead ones. An empty Lead battery has a fairly low internal resistance, and you may end up blowing the Li battery.

If you want to build a charger that uses a Li battery as a power source, that's perfectly doable, just be sure you stay below the maximum current you can draw out.

For the charge of the Li battery: here is a Link. Charging that type of battery seems a bit tricky, but I didn't fully read the article.

 

[diy50per]

Thank you all for the helpful advice. I decided against experiments with the large lithium ion batteries after all. As I am not doing such things on a professional level, it would not be worth the inherent risk.