parallel different packs

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goatman   10 kW

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parallel different packs

Post by goatman » Feb 01 2020 1:35pm

I have a 4p of 25r so 80amps and 10ah equals an 8c pack
I have a 3p of 18650pd so 30amps and 8.55ah equals a 3.5c pack

I run the 25r upto 10amps/4c BUT if I do the math of my max settings 2200 watts, dead battery is 51v so 43 amps, fresh battery at 70v is 30 amps so at WOT 4p is 7.5a-11amp per cell but in real riding I see 100 to 1000 watts/5amps or less or 2c. a 25r can take 4amps for rapid charging. 4p is 16 amps.

I want to parallel the 3p 18650pd using a 15 amp fuse. heres my thinking, 43amps is max draw and 7p is 6amps/cell and normal riding is 3amps or less/cell

john61ct   1.21 GW

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Re: parallel different packs

Post by john61ct » Feb 01 2020 2:33pm

Sorry but I find all that very hard to decipher.

If the two packs are the same chemistry cells, with the same **voltages** acceptable for LVC and charging

they can be paralleled no problem.

This will give potentially a higher amps discharge rate,

or a longer range, but likely not both.

Maybe better to be a bit more conservative about avoiding the knees for longevity.

And do regular capacity / ESIR benchmarking for EoL **separately**, as they will not be wearing out evenly.

Are BMS involved?

goatman   10 kW

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Re: parallel different packs

Post by goatman » Feb 01 2020 3:12pm

no bms but I manually balance, different chemistry, 25r is inr, 18650pd is imr.

im going for range, I was just going to do it but another post about paralleling cells of different voltages in a p group mentioned don't mix chemistries without using a fuse or resistor so I figure you can parallel two different packs if you protect the weaker pack with a fuse. so I made a 17s3p last night but thought id check here first before connecting.

I consider this other 3p pack would behave like regen or a charger if it has higher resistance than the 25r pack, the 25r does most of the work and the PD will keep balancing but I haven't a clue

another thread we were talking about putting 20 amps of regen into a 3p pack rated at 30 amp and 10ah, 10ah at 20 amp is 2c no problem

I couldn't find the ir of a Panasonic 18650pd. ill just do it and watch what the pack voltages are doing as I near the low voltage

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fechter   100 GW

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Re: parallel different packs

Post by fechter » Feb 01 2020 3:19pm

I did something similar on my Sur-ron but the cells are probably more similar between the two packs so it shares nicely. If there is a huge difference in internal resistance between the two packs, there will be a tendency for the stronger pack to do more work under high load then get recharged from the weaker pack during light load. Not necessarily a bad thing but charge shuttling will have some losses/heating. Opposite will happen during regen.

Also, make sure the two packs are very nearly the same voltage before joining. Within about 0.1v is plenty close enough.
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Sunder   100 MW

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Re: parallel different packs

Post by Sunder » Feb 01 2020 5:40pm

I echo Fetcher's post above, but add that since the cells are the same chemistry (or close enough), putting connections at the cell level would be preferred.

If you do the maths, even at differences of 2-3 tenths of a volt, the current between rebalancing cells is very low.
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goatman   10 kW

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Re: parallel different packs

Post by goatman » Feb 01 2020 6:28pm

the pack is kind of but not really individual fuse wired, its 18g copper to an 18g bus bar. I think 18g is 11amp optimal so I wanted to see what kind of heat that is and then ill try different fuse wires to see what kind of heat they make. and then fuse the pack

markz   100 GW

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Re: parallel different packs

Post by markz » Feb 02 2020 2:15pm

Paralleling different packs means when you connect them in parallel they should be very close in voltage otherwise one pack will instantaneously discharge into the other pack. 0.10V or 0.05V is a good difference.

Next you have to worry about draining one pack faster then the next. As an example, if one pack is 20V 5Ah and the other pack is 20V 7Ah and you parallel them, then the 5Ah pack will drain faster then the 7Ah pack, and it will drain it to an unhealthy state depending on the LVC of the controller.

Then you have to worry about the discharge rate for each pack, if the 5Ah is 10A(max) and the 7Ah is 18A(max) can only hope your controller keeps it to 10A.

Random numbers to get the point across.

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serious_sam   100 W

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Re: parallel different packs

Post by serious_sam » Feb 02 2020 5:20pm

markz wrote:
Feb 02 2020 2:15pm
Next you have to worry about draining one pack faster then the next. As an example, if one pack is 20V 5Ah and the other pack is 20V 7Ah and you parallel them, then the 5Ah pack will drain faster then the 7Ah pack, and it will drain it to an unhealthy state depending on the LVC of the controller.
At low discharge rates, they will drain at the same rate, because they are in parallel, and their voltages are held equal. i.e. their voltages will be the same, so their state of charge will be basically the same.

To reiterate what Fetcher said, at high discharge rates, the cells with lower internal resistance will carry more load, because their voltage will sag less.

It has nothing to do with rated capacity. If the 7ah pack has a lower internal resistance, then it will drain faster than the 5ah pack.

Once the load is removed, the higher internal resistance pack will discharge into the low resistance pack until voltages equalise again.

john61ct   1.21 GW

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Re: parallel different packs

Post by john61ct » Feb 02 2020 5:55pm

markz wrote:Next you have to worry about draining one pack faster then the next. As an example, if one pack is 20V 5Ah and the other pack is 20V 7Ah and you parallel them, then the 5Ah pack will drain faster then the 7Ah pack, and it will drain it to an unhealthy state depending on the LVC of the controller.

Then you have to worry about the discharge rate for each pack, if the 5Ah is 10A(max) and the 7Ah is 18A(max) can only hope your controller keeps it to 10A.

Random numbers to get the point across.
No that's all wrong. By definition being connected in parallel, both (tend to) stay at the same voltage.

If they are the same chemistry (similar C-rate capacity, similar voltage vs SoC mapping), then they will be drawn down evenly. More Ah comes from the bigger pack, proportionally less from the smaller, similar C-rates, no problem.
markz wrote:Paralleling different packs means when you connect them in parallel they should be very close in voltage otherwise one pack will instantaneously discharge into the other pack. 0.10V or 0.05V is a good difference.
In practice that delta times the number in series is safe enough. So yes 0.1V for 1S, but a volt or two for a 52V pack would be fine.


markz   100 GW

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Re: parallel different packs

Post by markz » Feb 02 2020 6:17pm

Well then I learned something new.
More Ah comes from the bigger pack, proportionally less from the smaller, similar C-rates, no problem.
Let me get this straight now.
Going to the extreme/physically-unattainable example of having a pack thats 20V(fully charged) and 1Ah paralleled with a pack that is 20V(fully charged) and 10Ah, you get 20V 11Ah. The 10Ah gets proportionally drawn down way faster then 1Ah which is very slowly drained.
Interesting.
Oh ok now I see it. The 10Ah pack just dumps electrons into the paralleled 1Ah and keeps the 1Ah topped up.

Its the series split that will drain the Ah faster, which is what my pack was/is. 10V in series with 10V nets 20V. In that case the matching of the voltages is not as critical in the initial connecting together, but if I had one pack that I did not charge and was say 5V plus the 10V pack, then 5V gets avalanched if used until it reaches LVC. I've had that happen to me. Heck even had a situation where I'd have 5S in series with 4S or 6S, but I only used 50% to stay safe.

goatman   10 kW

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Re: parallel different packs

Post by goatman » Feb 02 2020 6:17pm

ill keep my eyes on the voltages of the packs but I should be able to get a good idea about capacity and how the batteries are interacting with each other by measuring the watts and ah during recharging of each pack separately but they should be balancing each other as I ride.

I don't know about a controller regulating amps from separate batteries, theyre paralleled before reaching the controller, im using a 15amp fuse on the 3p to protect the batteries from each other, that's my starting test point, if Im pulling 45amps for 20 seconds over 7p about 6.5amps per cell or 19amp demand from the 3 p pack and the fuse doesn't blow ill know somethings not right but if it does blow, ill put in a 20 amp fuse and try again, if it doesn't blow I should be ok

I think its the resistance of the cells that regulate what flows through them. if im riding normal there shouldn't be any problems and the batteries should get along, the only time I can see a major issue happening is if im trying to pull 45 amps at the lvc and that should blow the 15amp fuse of the 3p pack

markz   100 GW

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Re: parallel different packs

Post by markz » Feb 02 2020 6:27pm

Yeah they equal out, but as long as the Ah are close then no worries.

john61ct   1.21 GW

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Re: parallel different packs

Post by john61ct » Feb 02 2020 11:12pm


markz wrote:Going to the extreme/physically-unattainable example of having a pack thats 20V(fully charged) and 1Ah paralleled with a pack that is 20V(fully charged) and 10Ah, you get 20V 11Ah.
Nothing unobtainable about that, at all?

> The 10Ah gets proportionally drawn down way faster then 1Ah which is very slowly drained.
Interesting.

No proportionally - as in SoC% - they could get drawn down at exactly the same rate, if the same chemistry, and IRL small differences won't matter.

Only in absolute terms - mAh or Ah per second - will the rate of discharge be different.

> The 10Ah pack just dumps electrons into the paralleled 1Ah and keeps the 1Ah topped up.

No, there will be no significant flow from one pack to the other, unless current demand is pushing up against one or the other's C-rate capacity, could just as easily be from the smaller to the larger.

In practice won't matter anyway, unless poor design or extreme edge cases.

Yes more precise matching of capacity and C-rate and volt/SoC mapping (SoH, resistance, aging etc) is critical with series'd modules.

3S + 6S + whatever S to get to a target voltage is not an issue, as long as each cell/group matches in the above criteria.


goatman   10 kW

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Re: parallel different packs

Post by goatman » Feb 03 2020 5:47pm

I did a test, I paralleled the packs and the smaller 3p pack had a 10 amp fuse so I had 4p on the bike and 3p in the backpack. I left with 69v and my phaserunner was set at 2500 watts wich is 36.23 amps divide by 7p is 5.17amps per cell. if I maxed the throttle I was pulling 36 amps and about 2200 watts. I couldn't blow the 10 amp fuse, I would go full throttle up the hill and come back down and repeat. 15kms later I still hadn't blown the fuse so I went inside and disconnected the batteries, tested with a DMM and the packs were 0.5v apart, the bike 25r was lower so I plugged them back in, hooked the phaserunner to the computer and upped the watts to 3500 watts. try again I blew the fuse right away at 2700 watts and I saw 50 amps so I put in a 15 amp fuse and I couldn't blow it. my volts were down to 62 and im not going to change my phaserunner amp settings so im charging the batteries instead to get my watts up. BUT after pulling 2700 watts and only about 1km traveled the batteries were 0.7v apart and when I left for the second test the batteries were only 0.1v apart. the batteries might drift to far apart in voltage at 2700 plus watts of discharge.

for me and my setup if I leave my settings at 2500 watt max and a 10 amp fuse I shouldn't have any problems but if I go over that I might have issues

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