Parallel battery charging?

Headdamage

New Member
Region
Canada
I'm intending to run a pair of matched 48v 10.5ah Lithium batteries in parallel. Each battery has it's own BMS (BMS SPECS = 15A RATED CONTINUOUS DISCHARGE, 50A MAX DISCHARGE, 5 AMP CHARGING CURRENT). I am intending to run them in parallel because I suspect that they are a little under rated for use with a BBSHD (correct me if I'm wrong) and that they would last longer and not be in danger of too high a discharge rate if run in parallel.

I'm familiar with, wiring and batteries but not with BMS equipped batteries. The question that I need answered is can both batteries be charged with one charger through one charging port while connected in parallel at their discharge ports or do I need to isolate and charge independently? Or can the charging ports be connected in parallel for charging (gut feeling is no)? Any other options?

Thanks for any help.
Parallel batts and BBSHD.jpg
 

Timpo

Well-Known Member
How are you connecting two batteries?
By using wires like this?

I heard you're supposed to use battery connectors like these, otherwise it's gonna cause problems.
 

Headdamage

New Member
Region
Canada
I was intending to use a simple parallel pigtail arrangement like the first one you list. However, the doohickey thing from Wattwagons is a rather elegant method but it doesn't appear to answer my charging question.
 

Timpo

Well-Known Member
I was intending to use a simple parallel pigtail arrangement like the first one you list. However, the doohickey thing from Wattwagons is a rather elegant method but it doesn't appear to answer my charging question.
Sorry, you're absolutely right, I did not answer the question. 😅
I don't think you can charge your battery from your discharge port though?

And somebody correct me if I'm wrong, but somebody here on EBR said that you can't just connect two batteries using pigtail cable posted (AliExpress link) above.
 

Timpo

Well-Known Member
For example, this bike has dual battery, but can NOT be used simultaneously.
There are a lot of cheaper ebikes with dual batteries that you can NOT use simmultaneously.

 

Timpo

Well-Known Member
Also I believe this guy didn't get a proper module (from WattWagons, Bolton, etc) so he can NOT use his batteries simultaneously.

 

harryS

Well-Known Member
Have you looked at a two port BMS? They're wired like this. SHown is a 36V pack, but a 48 is just the same, only with 3 more groups of series cells. The positive side of the output and charge connectors go to the positive terminal of the cells, but the negative side is connected to two separate solid state switches, controlled by X2 and X4.

X2 is for output and basically stays closed as long as all the cells are above minimum, X4 is for charging and stays closed if all the cells are below maximum, usually 4.2V. Both circuits also consider the current and will shut down if that's too high.
BMS_mos.jpg
So sure, it's safe to put two packs in parallel, as long as they're at the same voltage, The individual packs still have undervolatge shutdown and high current shut down. I do this often, Just make sure both packs are at the same voltage when you connect them.

So what about charging?

Draw the above diagram for two batteries, Replace X2 with a closed switch. I've looked at it. Here's my opinion. If you only use one charger, it will work but you will not have any overvoltage protection for the second battery. The current path flows thru the X2 path, This could be a problem if the second battery is unbalanced. Normally, when a battery is unbalanced, it stops charging when any series group hits 4.2V. Without overvoltage protection, if the first battery keeps charging, some cells in the second battery can be overcharged if it is unbalanced. That's bad. WIll wreck your battery. Maybe a fire.

What if you put a charger on both sides?. Problem doesn't go away. If the second battery is unbalanced, its second charger will stop charging it when any series group hits 4.2V. Nothing keeps the first charger from feeding the second battery. Same overcharge can happen.

So my opinion is you can hook them in parallel, but you have to break them apart for charging if you want to be safe.
 
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harryS

Well-Known Member
Of course, this raises the question ,,,what happens if you parallel an unbalanced battery with a balanced one? The latter will be a little higher than the first one, and it will charge it. Bummer, this never occurred to me.

I guess that's why to be safest, you have to use big power diodes to isolate the batteries from charging each other.
 

Headdamage

New Member
Region
Canada
Have you looked at a two port BMS? They're wired like this. SHown is a 36V pack, but a 48 is just the same, only with 3 more groups of series cells. The positive side of the output and charge connectors go to the positive terminal of the cells, but the negative side is connected to two separate solid state switches, controlled by X2 and X4.

X2 is for output and basically stays closed as long as all the cells are above minimum, X4 is for charging and stays closed if all the cells are below maximum, usually 4.2V. Both circuits also consider the current and will shut down if that's too high.View attachment 78881So sure, it's safe to put two packs in parallel, as long as they're at the same voltage, The individual packs still have undervolatge shutdown and high current shut down. I do this often, Just make sure both packs are at the same voltage when you connect them.

So what about charging?

Draw the above diagram for two batteries, Replace X2 with a closed switch. I've looked at it. Here's my opinion. If you only use one charger, it will work but you will not have any overvoltage protection for the second battery. The current path flows thru the X2 path, This could be a problem if the second battery is unbalanced. Normally, when a battery is unbalanced, it stops charging when any series group hits 4.2V. Without overvoltage protection, if the first battery keeps charging, some cells in the second battery can be overcharged if it is unbalanced. That's bad. WIll wreck your battery. Maybe a fire.

What if you put a charger on both sides?. Problem doesn't go away. If the second battery is unbalanced, its second charger will stop charging it when any series group hits 4.2V. Nothing keeps the first charger from feeding the second battery. Same overcharge can happen.

So my opinion is you can hook them in parallel, but you have to break them apart for charging if you want to be safe.
Your thinking here mirrors my own, the part that confuses me is that the shop I've bought them from says I can charge them together and I can't wrap my head around how this will work safely. I think I'll charge them separately unless I can find a good explanation as to why they can be charged together.
 

harryS

Well-Known Member
Doesn't take much work to use a Y-connector, and unplug it when charging. You'll take less time with two chargers too.

While it wouldn't take much effort to make a y-connector with diodes, I feel pretty safe connecting my small 36V packs together, There's typically about 1/4-1/2 volt difference between them, and very little current surges. With higher current packs, it might be hard to find diodes that can sustain the current and not cost you a volt of forward drop, My goals differ from yours. I run low current bikes with small batteries. I could just connect one pack as the first pack empties, but the bikes runs better with the batteries combined, and they last longer that way too,
 

PedalUma

Active Member
Region
USA
Simple is best. I sometimes run two batteries but that is only for rides over 50 miles. Just use one battery connector directly to the motor. Then swap batteries when you are ready of the second one. It takes two-seconds to swap batteries. I normally just take one battery to cut weight.
 

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fauconnier

New Member
Region
Canada
I would disconnect the Y connector and use 2 chargers for twice the speed, 2A bikes chargers are cheap. Although I use 2 Lipo balance chargers, It's what I do with my 2 x 12A 22.2v Lipo batteries in series bike. I very often use batteries in parallel in my RC heli and planes. Healthy similar batteries always discharge equally.
Because batteries sag, It's a lot better to use them in parallel than one after the other. More current (A) available for the ride and less sag for the batteries. A slow discharge rate is more desirable for battery longevity.
 
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PedalUma

Active Member
Region
USA
Thank you. You have a depth of understanding. Yes. Running two in Parallel is better because of sag. I just happen to like the simplicity of having one connector directly to the motor. This provides fewer wires, connections and complications. I also like two chargers. Over 90% of rides I just take one 3-lbs battery. If I need more juice I will take one slightly larger battery or use two small batteries sequentially.
 

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PedalUma

Active Member
Region
USA
Sorry, you're absolutely right, I did not answer the question. 😅
I don't think you can charge your battery from your discharge port though?

And somebody correct me if I'm wrong, but somebody here on EBR said that you can't just connect two batteries using pigtail cable posted (AliExpress link) above.
"I don't think you can charge your battery from your discharge port though?"
I have done this to jump the BMS on Specialized batteries that had shut down. We had a pallet of these duds. Then I could charge them the regular way.
 

harryS

Well-Known Member
I have three of those bottle packs. Two were GA cells and the thrid was generic chinese junk. After I pull 4AH of that one, it's pretty much gone. The other two deliver 8-9 AH. after two-three years. I found that the plastic water bottle cage would break after a few months. I bent up some u-brackets instead out of steel and attached them to my frames. This mini is fun. Only 35 pounds.

P1650108.jpg
 

PedalUma

Active Member
Region
USA
HarryS.,
Firstly, the plastic cages are junk and I only use strong MTB aluminum cages now. This is a side issue or question about battery and motor placement. I just weighed an eBike. It is 19.13Kg with accessories. Then I weighed the back and front separately, weighing them one foot off the ground with the other end on the ground using a bathroom scale on a milk crate. The total weight was 16.2Kg. That is about 15% less. I suspect that bikes will vary considerably on this measurement. And that this difference will help in better understanding handling characteristics. Many bikes will show less difference, some will show more. Riding with one wheel a foot higher than than the other is rare but will help show how a bike can go up and down surfaces such as potholes and curbs. I suspect that the bigger this difference the better the handling.
 

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