Parallel battery for extended range, is it safe?

[#boosted]

Long story short, Is it safe to run a different brand battery (both 18650 60v, newer packs, each have BMS in casing) in parallel for extended range?

If so, is it possible to run just ONE pack when needed or both when range is necessary? I can think of how to wire it up but i just dont know if this is a "no-no" in the lithium ion world... I would of course charge up the seperate battery and internal battery before reconnecting when needed.

 

[Triketech]

Not recommended as the different BMS's may have some oscillation between weaker/stronger cells.
Can leak smoke under extreme conditions.

 

[amberwolf]

If it's helpful there are several recent threads that cover various aspects of this that are in this list (which also includes irrelevant stuff, but you can generally see which ones I'm referring to by their titles) and many older threads as well:

https://endless-sphere.com/forums/search.php?keywords=parallel*&terms=all&author=amberwolf&sc=1&sf=all&sr=topics&sk=t&sd=d&st=0&ch=300&t=0&submit=Search

A better list might be this one by post instead of topic (again, not everything that shows up is relevant, but you can refine the search further if you like):
https://endless-sphere.com/forums/search.php?keywords=parallel*&terms=all&author=amberwolf&sc=1&sf=all&sr=posts&sk=t&sd=d&st=0&ch=-1&t=0&submit=Search


Some things to consider (that are covered in posts I've made in the various threads) are whether you're going to charge when paralleled (or only discharge), what kind of BMS is installed in each pack, state of charge and state of health of each pack, whether you need the paralleled system to accept regen braking charge, etc.

 

[tomjasz]

I found it best for me to charge packs separately and use a DPDT on-off-on the select between packs.

 

[#boosted]

I found it best for me to charge packs separately and use a DPDT on-off-on the select between packs.

This seems like the best option, switch to the second battery via mechanical switch if needed. You may not be able to get the benefits of lower discharge rate/battery sag but at least it seems to be 100% safe for both batteries.

 

[E-HP]

I found it best for me to charge packs separately and use a DPDT on-off-on the select between packs.

This seems like the best option, switch to the second battery via mechanical switch if needed. You may not be able to get the benefits of lower discharge rate/battery sag but at least it seems to be 100% safe for both batteries.

If you're going to consider a switch, then perhaps you'd consider making it all work automatically by adding a high current rectifier diode to the output of each battery. Then it doesn't matter what the charge state/level is on either since the diodes would take care of switching back and forth automatically.

https://www.amazon.com/LANTRO-JS-Rectifier-Batteries-Converter/dp/B094QRZSQC/ref=sr_1_2_sspa?crid=HT1KLKGLZBAL&keywords=rectifier+diode+50a&qid=1666825414

 

[tomjasz]

I found it best for me to charge packs separately and use a DPDT on-off-on the select between packs.

This seems like the best option, switch to the second battery via mechanical switch if needed. You may not be able to get the benefits of lower discharge rate/battery sag but at least it seems to be 100% safe for both batteries.

If you're going to consider a switch, then perhaps you'd consider making it all work automatically by adding a high current rectifier diode to the output of each battery. Then it doesn't matter what the charge state/level is on either since the diodes would take care of switching back and forth automatically.

https://www.amazon.com/LANTRO-JS-Rectifier-Batteries-Converter/dp/B094QRZSQC/ref=sr_1_2_sspa?crid=HT1KLKGLZBAL&keywords=rectifier+diode+50a&qid=1666825414

Interesting, but why?

 

[E-HP]

I found it best for me to charge packs separately and use a DPDT on-off-on the select between packs.

This seems like the best option, switch to the second battery via mechanical switch if needed. You may not be able to get the benefits of lower discharge rate/battery sag but at least it seems to be 100% safe for both batteries.

If you're going to consider a switch, then perhaps you'd consider making it all work automatically by adding a high current rectifier diode to the output of each battery. Then it doesn't matter what the charge state/level is on either since the diodes would take care of switching back and forth automatically.

https://www.amazon.com/LANTRO-JS-Rectifier-Batteries-Converter/dp/B094QRZSQC/ref=sr_1_2_sspa?crid=HT1KLKGLZBAL&keywords=rectifier+diode+50a&qid=1666825414

Interesting, but why?

I think the diodes would provide a similar benefit as direct paralleling, by treating the total capacity of both packs as one big battery. It won't have the increased current capability when one pack is charged higher than the other, but once both packs are close enough in voltage so the higher one's voltage sags down to the lower one's both packs will be contributing. The switch gives you the benefit of control, so you treat the packs separately and switch over once one is spent. That probably provides a better indication of what's left in the tank, so if you don't need the extra current, then a good option. With either setup, you can choose to run just one pack, and both also allow you to charge each pack separately.

 

[Diggs]

Just to add to the knowledge base. I've run parallel 52v packs for several years now. One is a 12 ah Luna Wolf pack that has dual XT90 output connectors. One of those connectors is connected to a generic 15 ah pack (18650) back in the trunk bag that has both charging and output on the single BMS port. I leave them connected during charging but usually only charge to 80% (90% if expecting to ride all day). Once a year or so I disconnect them from each other and charge each to 100% overnight for balancing and ride them down separately, then charge both to 80% and reconnect. I get almost no voltage sag (BBHSD) and best of all no range anxiety. Never had an issue but each to their own.