7s15p or 3*7s5p to quickly charge?

Batteries, Chargers, and Battery Management Systems.
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marine7   1 mW

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7s15p or 3*7s5p to quickly charge?

Post by marine7 » Feb 11 2020 3:49pm

Hey all,

Planning on building a battery pack for trolling motor, I will be using LG hg2 chem.
Battery model: LG HG2
Build: 7S15P or 3*7s5p

I would like to break the pack to 3 small packs to allow for fast charging, I would rather have a 3 charger to charge 7s5P instead of charging 7s15P it will take forever 45amp

Thinking of having 3 BMS for each pack separately and then connect the packs in parallel.
Would this be double without separating the pack every time I need to charge the 3 packs, my question can I attach the 3*7s5p together and have a break point in between when I charge them?
Is what Im thinking is crazy? Or double.

Sorry I’m not that guru but been building small packs of 3s for fish finder, and they work like a charm.

Many thanks

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Re: 7s15p or 3*7s5p to quickly charge?

Post by John in CR » Feb 12 2020 2:10am

Simply parallel the chargers instead. Then you don't have to reconfigure anything to charge.

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Re: 7s15p or 3*7s5p to quickly charge?

Post by john61ct » Feb 12 2020 3:04am

or just buy one big charger

no advantage to smaller packs

For best longevity stay below 0.5C though, unless you've got a compelling need to go faster.

Slower in cold weather or you can damage the cells.

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Re: 7s15p or 3*7s5p to quickly charge?

Post by dogman dan » Feb 12 2020 9:28am

15p,, one good charger,, one bms able to handle the big chargers output. That's ideal.

What I mean is,,, changing the p number does not change the individual cells recommended max charge rate. So 3p x max rate of each cell is not going to increase the cells max rate.

But depending on which bms you use, you might be limited to 5 amps charge rate for the pack. So if that is the case, 3 packs charging at 5 amps each will charge 3x faster than one pack, that is still limited to 5 amps charging rate.

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Re: 7s15p or 3*7s5p to quickly charge?

Post by 999zip999 » Feb 12 2020 11:32am

I break my 24s pack as two 12s for faster charging. No BMS. With BMS most BMS charge at 5 to 6 amps max. So all can be on one power strip at one time. What is the amp draw on your motor ? With a BMS I like on big pack. I would run one big 15p pack and charge at 10 to 12 amps no BMS only because it's a 45ah pack or so.

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Re: 7s15p or 3*7s5p to quickly charge?

Post by john61ct » Feb 12 2020 12:08pm

yes splitting high voltage **serial** connected packs up into lower voltage ones makes sense since the chargers are cheaper and better

But OP was talking about paralleled units, where no such advantage exists.

In fact paralleling down at the 1S level so there's just one or maybe 2 strings total is best

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Re: 7s15p or 3*7s5p to quickly charge?

Post by marine7 » Feb 12 2020 1:26pm

John in CR wrote:
Feb 12 2020 2:10am
Simply parallel the chargers instead. Then you don't have to reconfigure anything to charge.
Wouldn't it be easier to just buy big charger?

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Re: 7s15p or 3*7s5p to quickly charge?

Post by marine7 » Feb 12 2020 1:34pm

john61ct wrote:
Feb 12 2020 3:04am
or just buy one big charger

no advantage to smaller packs

For best longevity stay below 0.5C though, unless you've got a compelling need to go faster.

Slower in cold weather or you can damage the cells.
So the pack is 7s*15p= 45amp and each cell when it discharge at rate of 1amp it gives 3000mha
The motor takes about 15amp continuous and peak is 18amp

with .5c per cell that would mean .5*3000=1.5amp per cell ,now do I multiply by the Series 7*3000 and then multiply *.5= 10.5 amp :shock:
Sorry I've been reading the C-rate for a while but can't get the grip of it.
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Re: 7s15p or 3*7s5p to quickly charge?

Post by marine7 » Feb 12 2020 1:40pm

999zip999 wrote:
Feb 12 2020 11:32am
I break my 24s pack as two 12s for faster charging. No BMS. With BMS most BMS charge at 5 to 6 amps max. So all can be on one power strip at one time. What is the amp draw on your motor ? With a BMS I like on big pack. I would run one big 15p pack and charge at 10 to 12 amps no BMS only because it's a 45ah pack or so.
My motor will draw continuous at full speed 15amp where most of the time will be about 10amp,
Yeah I noticed the C charging rate would about 10.5amp which would take about 4.5 hours or so.

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Re: 7s15p or 3*7s5p to quickly charge?

Post by marine7 » Feb 12 2020 1:44pm

john61ct wrote:
Feb 12 2020 12:08pm
yes splitting high voltage **serial** connected packs up into lower voltage ones makes sense since the chargers are cheaper and better

But OP was talking about paralleled units, where no such advantage exists.

In fact paralleling down at the 1S level so there's just one or maybe 2 strings total is best
It started to make sense to me, thanks for the heads up.

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Re: 7s15p or 3*7s5p to quickly charge?

Post by john61ct » Feb 12 2020 4:16pm

Sorry can't make sense of that. Amps is not a unit of capacity, Ah is.

Start with the specs for your cell.

7S * (3.6?) gives your voltage

what is the mAh per cell?

times 15P gives Ah capacity.

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Re: 7s15p or 3*7s5p to quickly charge?

Post by John in CR » Feb 12 2020 7:39pm

C rate is simple, which is how fast you can charge or discharge your batteries. 1C is a full discharge in one hour and .5C is the rate of current for 2hrs. . You have 3000mAh cells (3Ah cells), so a 1C charge or discharge per cell in parallel is 3A. You have 15 in parallel, so 45ah of capacity, making your 15A discharge current only at 0.33C, since it would take 3 hours for a full discharge...nice and conservative.

For charging, your batteries are rated for a 0.5C standard charge rate, or 1.5A per parallel cell or 22.5A for the pack. Personally I would stay well below that for the sake of battery life. I'd stick to 5 or 6A max as a standard charge rate unless you really need faster charging than that on a regular basis. Manufacturers try to rate their cells as high as they can, because they don't really care if your battery decreases in capacity by 20% after 500 cycles. They'll gladly sell you more.

That's why they say to charge to 4.2V per series cell despite the fact that test have shown that charging to only 4.0-4.1V combined with discharging only to 20% or so of remaining capacity increases cell life drastically. Electric car manufacturers understand this and restrict charge/discharge voltages in that manner in order to have warranties of 8-10years or 100,000 miles on their battery packs.

Other things you want to do to extend the life of your battery pack is avoid charging a very warm pack, and avoid charging and leaving your pack at full voltage all night as part of your regular routine. That's why it's good to charge at the lowest current you can that fits your routine. Then for the times you need a faster charge add another charger or two in parallel. It's good to have at least one backup charger anyway, so a charger failure doesn't leave you without your ebike transportation.

NOTE THAT NOT ALL BATTERY CHARGERS CAN BE CONNECTED IN PARALLEL. They must be isolated chargers with both wires of the output isolated from the input. It was irresponsible of me before to just make the blanket suggestion to put chargers in parallel. I've been using only isolated chargers for a long time and parallel them on a regular basis.

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Re: 7s15p or 3*7s5p to quickly charge?

Post by marine7 » Feb 12 2020 8:52pm

john61ct wrote:
Feb 12 2020 4:16pm
Sorry can't make sense of that. Amps is not a unit of capacity, Ah is.

Start with the specs for your cell.

7S * (3.6?) gives your voltage

what is the mAh per cell?

times 15P gives Ah capacity.
Yeah I need to stop mixing the terminology

3000mAh
3.6V nominal
15p=45Ah

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Re: 7s15p or 3*7s5p to quickly charge?

Post by marine7 » Feb 12 2020 8:57pm

John in CR wrote:
Feb 12 2020 7:39pm
C rate is simple, which is how fast you can charge or discharge your batteries. 1C is a full discharge in one hour and .5C is the rate of current for 2hrs. . You have 3000mAh cells (3Ah cells), so a 1C charge or discharge per cell in parallel is 3A. You have 15 in parallel, so 45ah of capacity, making your 15A discharge current only at 0.33C, since it would take 3 hours for a full discharge...nice and conservative.

For charging, your batteries are rated for a 0.5C standard charge rate, or 1.5A per parallel cell or 22.5A for the pack. Personally I would stay well below that for the sake of battery life. I'd stick to 5 or 6A max as a standard charge rate unless you really need faster charging than that on a regular basis. Manufacturers try to rate their cells as high as they can, because they don't really care if your battery decreases in capacity by 20% after 500 cycles. They'll gladly sell you more.

That's why they say to charge to 4.2V per series cell despite the fact that test have shown that charging to only 4.0-4.1V combined with discharging only to 20% or so of remaining capacity increases cell life drastically. Electric car manufacturers understand this and restrict charge/discharge voltages in that manner in order to have warranties of 8-10years or 100,000 miles on their battery packs.

Other things you want to do to extend the life of your battery pack is avoid charging a very warm pack, and avoid charging and leaving your pack at full voltage all night as part of your regular routine. That's why it's good to charge at the lowest current you can that fits your routine. Then for the times you need a faster charge add another charger or two in parallel. It's good to have at least one backup charger anyway, so a charger failure doesn't leave you without your ebike transportation.

NOTE THAT NOT ALL BATTERY CHARGERS CAN BE CONNECTED IN PARALLEL. They must be isolated chargers with both wires of the output isolated from the input. It was irresponsible of me before to just make the blanket suggestion to put chargers in parallel. I've been using only isolated chargers for a long time and parallel them on a regular basis.
Appreciate your time, well written and easy to understand.
It was irresponsible of me before to just make the blanket suggestion to put chargers in parallel.
I would always do more research before trying anything for sure but great to know.

Do you have a link to read about parallel charging or a place to read more about it. how to connect them to the parallel cells etc.

I remember reading about TP4056 but that all for individual cells not more than 1s

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Re: 7s15p or 3*7s5p to quickly charge?

Post by john61ct » Feb 12 2020 9:02pm


marine7 wrote: 3000mAh
3.6V nominal
15p=45Ah
So 25V, go from 10A to 15A charging, even up to 20A would be OK for a 2-hour charge

If your setup is safe for overnight as low as 5A.

The 15A motor load gets up to 0.33C only, so very low,

don't discharge too low, keep charging under 29V, should get very long lifespan.



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Re: 7s15p or 3*7s5p to quickly charge?

Post by amberwolf » Feb 13 2020 2:12am

John in CR wrote:
Feb 12 2020 7:39pm
NOTE THAT NOT ALL BATTERY CHARGERS CAN BE CONNECTED IN PARALLEL. They must be isolated chargers with both wires of the output isolated from the input. It was irresponsible of me before to just make the blanket suggestion to put chargers in parallel. I've been using only isolated chargers for a long time and parallel them on a regular basis.
parallel chargers don't have to be isolated, as long as the input line that isn't isolated from the output ground is the *same* input line on them all.

series chargers do. (otherwise you short across the "lower" charger, the most negative one, if the output ground is common to one of the input lines)


it can still be true that not all chargers can be paralleled; it depends on the design of their output stages. i can't think atm of the problematic design issue, though. :oops:

but most of them should work fine this way, though i've not paralleled any personally. (i've only paralleled assorted current-limiting power supplies, either as chargers or as power supplies).

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Re: 7s15p or 3*7s5p to quickly charge?

Post by John in CR » Feb 13 2020 5:00am

Thanks Amberwolf, I don't know what I was thinking.

Regarding paralleling chargers, I just make up an adapter to fit my bike-side plug with multiple charger connectors on the other end. As long as the bike connector can handle the total current it's quite simple. It doesn't even matter if chargers don't cut off at exactly the same voltage as long as they are all below your battery packs max. In fact mine have different cutoffs. I have one charger for very full charges before occasional long rides, and my other chargers cut off much lower for daily use.

IMO 29V is too high for 7s for every day use if you're really interested in battery life. 28V-28.2V will give your pack a much longer service life. The car companies have really done their homework and their over-engineered solutions are good to follow. 45ah is a nice large capacity pack, so there's no reason to charge to high voltage other than occasionally to force a non-programmable BMS to balance. While I've gotten longer than rated life charging to 4.15V/cell, but now that I'm charging to 4.0v-4.05v/cell I'm seeing much slower degradation of capacity. Another benefit of very conservative charge voltage is that the pack would need to go much further out of balance before damaging cells.

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Re: 7s15p or 3*7s5p to quickly charge?

Post by marine7 » Feb 13 2020 1:23pm

Im more likely to go with smart BMS and go below 4.1V as I would not need that longevity of the extra charge.

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Re: 7s15p or 3*7s5p to quickly charge?

Post by 999zip999 » Feb 13 2020 5:20pm

Have you purchased the cells yet ? You could use used auto grade leaf modules 4:45 amp your hours or another automotive grade patent cans terminals are there easy peasy do you know how many connections that are on a 7s 15p pack ? Lots. Over 200.

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Re: 7s15p or 3*7s5p to quickly charge?

Post by 999zip999 » Feb 14 2020 2:22pm

There are used leaf modules in the used for sale section. It's worth a look. I have not use them. Oh just saw your are in New Zealand

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Re: 7s15p or 3*7s5p to quickly charge?

Post by marine7 » Feb 18 2020 9:26pm

999zip999 wrote:
Feb 13 2020 5:20pm
Have you purchased the cells yet ? You could use used auto grade leaf modules 4:45 amp your hours or another automotive grade patent cans terminals are there easy peasy do you know how many connections that are on a 7s 15p pack ? Lots. Over 200.
I have not got the cells yes, Im more likely to lean towards the harvested LGGB or LGDB with 3200mha.

I'm not sure what do you mean by lots over 200, if its 7s=7cells in one row and 15p= 15*7 = 105 cells? they're definitely a lot but manageable size.

I will have a look on the sale section thank you.

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Re: 7s15p or 3*7s5p to quickly charge?

Post by 999zip999 » Feb 18 2020 10:05pm

I like big ah cells and if 45ah there are automotive
grade cells in cans as 45ah and others. I mean a boat so less connections the better. Or lots of spot welds more possible for corrosion at the weld point with moisture and or salt water. I think leaf has 45ah cans ect.
Or do you have access to good use balance cells ?

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Re: 7s15p or 3*7s5p to quickly charge?

Post by 999zip999 » Feb 18 2020 11:35pm

viewtopic.php?f=14&t=89680 maybe this can help.

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