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Can I wire two Pre built 36v packs in series to make 72v

Posted: Nov 26 2019 7:10pm
by dubinaz225
I have to 36 volt 12 amp hour 18 650 battery packs each one has a BMS built into it. If I charge them separately as 36v packs is it possible to wire them in series and make a 72v battery .. will the BMS have a problem ?

Re: Can I wire two Pre built 36v packs in series to make 72v

Posted: Nov 26 2019 7:26pm
by E-HP
dubinaz225 wrote:
Nov 26 2019 7:10pm
I have to 36 volt 12 amp hour 18 650 battery packs each one has a BMS built into it. If I charge them separately as 36v packs is it possible to wire them in series and make a 72v battery .. will the BMS have a problem ?
Ideally the two pack have had about the same use, so have the same capacity. If so, there shouldn't be an issue, but if one of the two BMSs trips when the packs get low, you should turn off the bike start pedaling/pushing.

Re: Can I wire two Pre built 36v packs in series to make 72v

Posted: Nov 26 2019 9:21pm
by john61ct
Depends on the BMS and how you wire it.

As stated total range will be dictated by the weakest sub-pack.

If you don't **need** the higher voltage, paralleling is less fraught.

Re: Can I wire two Pre built 36v packs in series to make 72v

Posted: Nov 26 2019 10:08pm
by Voltron
Like stated already, maybe, depending on the BMS.
But the concept is just fine... I run several 36v doubled up packs. It's a great way to give 72v to a motor that could use some more volts for some more rpms with cheap, easy to get batteries and chargers. Now 72v is getting more available commercially, but that's a pretty recent thing.

Here's my heavy weight set up... 6 36v batteries in series for 3 72v modules, then paralleled up. I can also run off of just one module for light weight short range sprints.
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I used to get lots of Pedego batteries from a recall, so the silver box at the bottom is a regular one still in it's shell, then a matching one out of its box paired up on the lower rack for 72v 14ah. Then 2 more that got deboxed, then split into long skinny shapes instead of boxy, but it's the same Pedego batteries as the bottom set for another 14ah.
The top set is a pair of Bosch batteries, also uncased, that I got for free because of defective BMSs, that are 10ah.

So that's a little over 3100 watt hours right off the charger, from 6 free batteries that would have gotten thrown away! 😀

The pack looks huge (and it is!) but one plus I didn't anticipate was adding the last top module made it just tall enough to lay my chest on for long aero tuck sessions..so comfy suddenly!

Re: Can I wire two Pre built 36v packs in series to make 72v

Posted: Nov 26 2019 10:16pm
by Voltron
Another plus is charging, 36v chargers are cheap and easy to get. And if you're in a hurry, easy to drop the time by running a charger to each sub pack. This was for 2 modules.... But I did spend a good bit of time tweaking the chargers to get exactly matching voltage cutoff, so the sub packs would all be the same voltage during hookup.
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Re: Can I wire two Pre built 36v packs in series to make 72v

Posted: Nov 27 2019 11:43am
by HK12K
Ideally you'd want the cells in both packs to be as closely matched as possible. Even if they both contain the exact same brand and model cells, if one pack is well worn or has high ir, and the other is new with low ir, putting them in series will result in the weaker packs cells being put under a lot of stress and may kill them sooner than they'd otherwise die, afaik. Of course with enough strings in parallel the demands on the pack lessen so this may or may not present itself as an issue for some of us, but if you're planning on pushing the packs hard it just might be worth considering.

Re: Can I wire two Pre built 36v packs in series to make 72v

Posted: Nov 27 2019 2:30pm
by john61ct
With serial'ing that matching issue becomes **very** important, not just "ideally".

As above, the weakest sub-pack will determine, limit the performance of the whole

Re: Can I wire two Pre built 36v packs in series to make 72v

Posted: Nov 27 2019 2:50pm
by E-HP
john61ct wrote:
Nov 27 2019 2:30pm
With serial'ing that matching issue becomes **very** important, not just "ideally".

As above, the weakest sub-pack will determine, limit the performance of the whole
The OP could also monitor both packs and make adjustment accordingly or shut down the bike if either pack is sagging too low under load. I'm going to be using a couple of these to monitor each pack when I add a series lipo pack to my current battery:
https://www.amazon.com/gp/product/B07MX ... K2VM&psc=1

Since my add on pack will be lipo, and 4s, I could also use one of these LV alarms:
https://www.amazon.com/Battery-Monitor- ... =hi&sr=1-4

Re: Can I wire two Pre built 36v packs in series to make 72v

Posted: Nov 27 2019 4:39pm
by john61ct
What adjustments?

Yes of course you need to monitor and stop if you get too low.

SoC primarily, counting Ah, with LVC as a failsafe backup.

My point is, in parallel the strongest pack is still contributing once the weaker ones have stopped.

Serial'd, everything stops soon as the weakest is done.

Re: Can I wire two Pre built 36v packs in series to make 72v

Posted: Nov 27 2019 4:53pm
by HK12K
Bingo.

Re: Can I wire two Pre built 36v packs in series to make 72v

Posted: Nov 27 2019 5:13pm
by E-HP
john61ct wrote:
Nov 27 2019 4:39pm
What adjustments?
I mean riding adjustments to keep from sagging too low, but still be able to get home. In other words, using the grey matter to make choices using the data, rather than relying on electronics to make them. Pretty much everyone here is smarter than a controller, given the appropriate data for decision making. Not arguing parallel vs serial, but using the grey matter most people can deal with the limitations of using a serial set up. (not saying everyone)

Re: Can I wire two Pre built 36v packs in series to make 72v

Posted: Nov 27 2019 6:29pm
by john61ct
I see, you mean as in pedalling harder, walking up hills, like that?

Re: Can I wire two Pre built 36v packs in series to make 72v

Posted: Nov 27 2019 6:46pm
by E-HP
john61ct wrote:
Nov 27 2019 6:29pm
I see, you mean as in pedalling harder, walking up hills, like that?
Sure, if you have to or don't plan well. Everyone has to learn the capability of their battery pack, even if it's just a single pack. More to keep track of with packs in series, but not rocket science. Take in the data and adjust accordingly. If you screw up, take note for next time. The biggest safeguard for an ebike is between our ears.

Re: Can I wire two Pre built 36v packs in series to make 72v

Posted: Nov 28 2019 6:22am
by dubinaz225
Both packs are identical . Both use 18650 Panasonic cells and have similar amount of cycles..

Re: Can I wire two Pre built 36v packs in series to make 72v

Posted: Nov 28 2019 10:39am
by HK12K
dubinaz225 wrote:
Nov 28 2019 6:22am
Both packs are identical . Both use 18650 Panasonic cells and have similar amount of cycles..
:thumb:

Re: Can I wire two Pre built 36v packs in series to make 72v

Posted: Nov 28 2019 12:20pm
by 999zip999
After 1/2 down take a voltage read on the packs and see what you get. If even use to 2/3 and get a reading is one stronger than the other ?

Re: Can I wire two Pre built 36v packs in series to make 72v

Posted: Nov 28 2019 1:47pm
by 999zip999
I would just use 9-10ah and see how it works out

Re: Can I wire two Pre built 36v packs in series to make 72v

Posted: Nov 28 2019 11:52pm
by Voltron
The OP didn't say exactly why he wanted to go 72v, but if it's to get more rpm out of his motor, more ah won't help.

And his 36v packs are 12ah, so not sure how a 9-10ah one will help?

Re: Can I wire two Pre built 36v packs in series to make 72v

Posted: Nov 29 2019 10:52am
by HK12K
Voltron wrote:
Nov 28 2019 11:52pm
The OP didn't say exactly why he wanted to go 72v, but if it's to get more rpm out of his motor, more ah won't help.

And his 36v packs are 12ah, so not sure how a 9-10ah one will help?
I believe he was suggesting that the op connect everything and then check his per cell voltages after riding around and consuming 9-10 ah or so, to see how far out of balance things may or may not have gone.

Re: Can I wire two Pre built 36v packs in series to make 72v

Posted: Nov 29 2019 10:55am
by Voltron
That makes more sense 👍