paralleling in battery packs.

[Waynemarlow]

Whats the easiest and correct way to parallel in 2 unequal capacity packs of batteries that may have different voltages. We have some 350Wh 52 volt packs which are our main EBike battery. Of late we have been doing longer and more energy sapping routes that the 350Wh packs are not sufficient.

We carry a small get me home pack in our rucksacks just for this and to date we simply remove the main battery and replace it with the small one. Its a pain and it would be far easier to simply plug in the smaller pack somewhere.

Thanks.

 

[amberwolf]

Lots of threads on exactly this kind of topic; many of the ones listed here with appropriate titles will help:

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

 

[Jan-Erik-86]

By different voltage, do you mean different state of charge, or different types of battery packs (like 13s vs 14s)? If only different state of charge, simply change both packs to within 1v different, then connect them together and it will work fine for both charge and discharge (assuming both have a bms with same charge /discharge port. If not, they should be charged separately). The capacity difference does not matter in parallel.

 

[john61ct]

Best is to just connect everything you got at the same voltage from the beginning.

You are not achieving anything useful by keeping them separate and in fact reducing your total capacity by a bit (see Peukert's Law).

Just track your Ah consumption so you know when to start limping home at the most efficient speed

 

[Waynemarlow]

Its sort of that simple but also a little more complicated. We have to do quite a few lifts over stiles and gates and such like to access the best areas as there's a bit of grey area here in the UK with whats allowed and not allowed on the paths. Also we get trees down and streams to cross which means its much easier to carry one pack in your rucksack until its actually needed. Lifting a 25kg bike constantly is a pain and yet 22kgs just seems manageable.

I was on the understanding that 0.1 volt difference was probably max between packs if you are simply paralleling them in, putting a big chunky diode loses 0.6 volts and a simple switch would have to carry `at least 15 amps which is a big switch.

 

[goatman]

i use a fuse and have connected with 0.7v difference but do your own testing

https://endless-sphere.com/forums/viewtopic.php?f=6&t=104027&p=1556192&hilit=marin#p1527424

 

[Jan-Erik-86]

1v difference is fine on a pack like that, you can probably go much higher than that also without any risks. The current flowing from one battery to another is very low as its limited by the voltage difference x battery pack resistance (including all the cables etc).
Think about how much current you need to pull to get only one volt battery sag, that's the ~same current that will flow between the batteries, and only for a short time. There might be a very short current spike higher than that, but that shouldn't cause any issues.

Ive had over 2v differences when parallelling my two previous 13s packs, and the current flow was still very low and not enough to trip the 30a fuse or the 35a bms.

 

[john61ct]

I was on the understanding that 0.1 volt difference was probably max between packs if you are simply paralleling them in

0.1V times the number of cell-groups

So 14S, 1.4V pack voltage difference is NP.

 

[Waynemarlow]

Ah that makes more sense.

OK but I would like to be changing over from an almost fully discharged battery 39 V to fitting a 58.4 volt fully charged battery.

Somehow I need to reliably be able to switch from one battery to the other without the need of a large and heavy duty switch that will isolate the other battery when its changing over.

Thanks for your interest guys.

 

[amberwolf]

OK but I would like to be changing over from an almost fully discharged battery 39 V
to fitting a 58.4 volt fully charged battery.

If that's a 14s battery, based on the 58.4v full discharging it down to 2.7v/cell isn't really a great idea. Uusally about 42-45v is a more normal cutoff for a 14s battery.

Somehow I need to reliably be able to switch from one battery to the other without the need of a large and heavy duty switch that will isolate the other battery when its changing over.

Uh, so you want to parallal a 39v battery with a 58.4v battery? That's the only way to avoid using a switch and not disconnect the empty pack first, and then just plug in the full one.

You do realize you're basically shorting the full battery to the empty one, and charging at a very high current rate thru the discharge port of the BMS, which means you're charging thru the body diodes of the FETs if the BMS has switched them off due to LVC? It could just blow the FETS on the BMS, or other various nasty results you don't want.


Otherwise you'll have to unplug the empty one, then plug in the full one.


If you want to literally switch from one to the ohter, with both wired to the bike already, the switch must be able to handle the full discharge current you will ever pull thru the battery, and it also has to handle the voltage difference between the pack being switched to, and the present capacitor voltage of the controller, which will be whatever the empty pack was at.


It's a hell of a lot simpler to either just wire them both in parallel and run them that way (see the link I posted originally), or to just swap them out as needed, taking off the empty and plugging in the full.

 

[Waynemarlow]

If that's a 14s battery, based on the 58.4v full discharging it down to 2.7v/cell isn't really a great idea. Uusally about 42-45v is a more normal cutoff for a 14s battery.

Otherwise you'll have to unplug the empty one, then plug in the full one.

If you want to literally switch from one to the ohter, with both wired to the bike already, the switch must be able to handle the full discharge current you will ever pull thru the battery, and it also has to handle the voltage difference between the pack being switched to, and the present capacitor voltage of the controller, which will be whatever the empty pack was at.

It's a hell of a lot simpler to either just wire them both in parallel and run them that way (see the link I posted originally), or to just swap them out as needed, taking off the empty and plugging in the full.

Sorry yes we have the cutoff set at 44 volts and not 39 V ( 13 S battery )

Yes starting to think we might have to continue to manually unplug and replug in the new battery, it only takes a few minutes and keeps the bike weight down.

The likes of Specialized do have small batteries which they plug in at any stage. I guess they must have some sort of electronics on board to handle this and that's extra weight and complexity.
https://www.specialized.com/it/en/sl-range-extender-battery/p/173733

Thanks

 

[amberwolf]

Sorry yes we have the cutoff set at 44 volts and not 39 V ( 13 S battery )

A 13s battery only charges to around 54v, not 58v. So are you using a 13s battery *and* a 14s battery? Or do you just need to get all your numbers straight *before* you post?

If youre using two different voltage batteries, you definitely can't parallel them directly, and would need to switch between them either manually or electronically.

The likes of Specialized do have small batteries which they plug in at any stage.

What do you mean by "small batteries which they plug in at any stage", *exactly*?

 

[Waynemarlow]

Sorry you beat me to the reply

https://www.specialized.com/it/en/sl-range-extender-battery/p/173733

These can be plugged in at any stage and you can set up the battery to be fully in parallel from the start, to be plugged in at any stage and plugged in and used exclusively as in remove the main battery for on board aircraft.

 

[Waynemarlow]

Sorry yes we have the cutoff set at 44 volts and not 39 V ( 13 S battery )

A 13s battery only charges to around 54v, not 58v. So are you using a 13s battery *and* a 14s battery? Or do you just need to get all your numbers straight *before* you post?

If youre using two different voltage batteries, you definitely can't parallel them directly, and would need to switch between them either manually or electronically.

The likes of Specialized do have small batteries which they plug in at any stage.

What do you mean by "small batteries which they plug in at any stage", *exactly*?

We do use different voltages between our group, most are on 48 volt batteries, I've gone over to 52 volts to see if they can be used reliably. We have set up the motors to handle either as we may get into a situation where we need to use anothers battery to get home. Under normal circumstances a like for like battery would be changed over.

 

[john61ct]

I would shut everything down, remove the depleted pack, insert the fresh one, then start everything back up.

KISS

 

[Waynemarlow]

Yes that's what we do already, but it would be nicer if we could have the main battery as a semi permanent, frame mounted, that we didn't have to remove when we put the 2nd battery on.

Hey ho, what we do already is not that inconvenient, so we'll stick to doing what we already do.

Thanks All.

 

[john61ct]

Are you 100% sure when you connect the new Full to the old empty, there is no flow of energy from the former into the latter?

Allowing that would waste a lot.

 

[Waynemarlow]

Yes the voltage difference is going to be some 14 volts and as far as I'm aware you could damage the cells or BMS if we were to simply parallel in the batteries.

My original thought was that there maybe some small piece of electronics or simpler diode based way of isolating each battery from each other without the loss of too much voltage, certainly the likes of the packs Specialized are using must have this capability.

The other basic way would be to find a 2 pole connector that could handle 20 amps and that would isolate the original battery as you plugged it in. Any ideas ?