Battery questions - Higher Nominal Voltage and Series Balancing...

Hey, I bought 16x 3.6V 60aH LG cells. Originally, I wanted to do a 60V (57.6V) ebike so I can hit mid, upper 30s for top speed. Then, I found it is hard to find different PAS level ebike kits in this COVID-19 era for 60V fat tire ebike and 60V is just not a popular ebike voltage.

I also want to eventually upgrade to 72V+ (I have 72V cyclone atm) and this is also on a fat tire bike. My latest thought is to use 14x of the cells and do 52V and eventually upgrade to 21x of the cells to do a 75.6V on a 72V kit. I know I will lose some range due to controller voltage levels. But, this is so I can make the cells into modular 7s packs = 2x is 52V and 3x is 75.6V and 4x is 100V AND for it to fit into the bike frame.

1. Anyone see any problems running a 75.6V nominal battery charged to 84V on a 72V kit? Will the 72V charger be able to charge the 75.6V nominal pack to 84V? I think so but wanna ask

2. So, apparently, there might be a concern on balancing smaller bms battery packs (25.2V in this case) in a series configuration. For the 52V ebike, I will make 2x 7s1p packs with their own BMS and then wire this in series to make the 52V battery. It will be charged the same way 52V series down to the 25.2V packs. I thought this was okay to do but maybe not? It will become unbalanced? I dont see how as each pack will have their own BMS and the 52V series connection will split 52 into 26V for each pack? Maybe, this becomes more a concern with 3 or 4 series packs?

Thx

First off, whatever sub-pack voltage you use, keep it under 10S, even 8S, gives you lots of choices to use reasonably priced RC "hobby chargers", or dedicated balancers,

do a great job balancing compared to 99.9% of BMS out there.

In fact you could buy "protective-only" BMS with no balancing function.

You would be well served by the Grin Satiator, well worth the extra price

**if** you want to charge at full-pack level voltages, no need to break the intra-sub-pack serial connections every time.

The #7205 "36V-84V" outputs 36-103V (manual states 20-103V??) at 4-5A, multiples can be stacked for faster higher current.

Or, you can meantime get away with just charging at the 7S sub-pack level with hobby charger(s), could rebalance or not per cycle choice (not is faster).

But that means breaking the serial connections if you want to do more than one pack at a time, unless you are **100% sure** the chargers are electrically isolated from the AC side and each other.

bakaneko said:

I also want to eventually upgrade to 72V+ (I have 72V cyclone atm) and this is also on a fat tire bike. My latest thought is to use 14x of the cells and do 52V and eventually upgrade to 21x of the cells to do a 75.6V on a 72V kit.

Click to expand...

So 21S is nominally 75-78V, about midpoint 50% Soc at rest.

A BMS might require 88V CV setpoint, held for a long time, but that is stressful, bad for longevity.

85-87V is gentle enough but 84V will not lose much range either

and staying under that voltage certainly should not cause any problem for a stock even cheapo controller labelled for 72V use.

Others familiar with your specific controller can advise about the range up to 87V if you need to get every last mAh stuffed in there.

Of course having your sub-packs set up for different Ah capacities, paralleling as well as serial'ing, that gives flexibility for long vs short trips.

As to that last, lots of variables there.

Already mentioned not using BMS for balancing, more generally just keep each sub pack balanced within itself is fine.

They should all be at the same pack-level voltage / SoC before assembling into a full pack.

So long as you know these two factors are OK, you really do not need each sub-pack to have their own BMS.

While in use, there is no need for HV protection.

Only LV, and that can be at the pack level voltage, so long as the cells / groups / sub-packs remain close in capacity

you just need to track / calibrate, at what pack voltage does the weakest cell / group (1S voltage) hit your desired lowest-limit floor?

Or you could just use one BMS, designed for your pack configuration (e.g. 21S) with the balance leads wired with multi-pin plugs so as to be easily removed as needed.

The number of sub-packs is not an issue, but staying below 8S give lots more choices in balancing chargers, and/or the dedicated balancers much less costly.

thx for feedback, my dude.

There is no such thing as series balancing. A parallel group can be balanced together, even if it is not the ideal. In a series, each cell is balanced and monitored individually. Then, lico chemistry doesn’t need balancing if you avoid discharging lower than 3.7v, thus building a battery that has extra capacity for your usual range is letting you bulk charge most of the time, and balance only once or twice a year.

Balancing is balancing. Parallel does it "naturally", once in series then a device is needed.

Cells are very rarely balanced and monitored individually in the use cases here, (although I agree that is the ideal) usually each group (in parallel at 1S voltage) gets treated as if it is one big cell.

Many make packs from mixed salvaged cells, but even a perfectly matched bank when new, once it ages enough may require more frequent and higher-amp balancing

independent of the specific chemistry selected.

yeh, its a 3kWh pack. I am going to do two 7s1p in series to make 52V nominal. I will attach a 100A power meter to each pack and detach the series connection and parallel them to balance the packs (>0.75V). As you mention, I doubt they get out of balance but just in case I will monitor them. I know there are series balancers out there but maybe another time.

john61ct said:

Cells are very rarely balanced and monitored individually in the use cases here, (although I agree that is the ideal) usually each group (in parallel at 1S voltage) gets treated as if it is one big cell

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That is how all Chinese batteries are built, and a good reason not to buy them. When you build your own, with lico chemistry especially, you don’t need to reproduce this old, stupid, cheap and dangerous method. That was acceptable with safer chemistries but lico, even in the form of compressed round cells, is NOT a safe chemistry.

LiCo cells need to be monitored individually, and balanced individually when required. Your own battery assembly should be better than Chinese, if you will be running high Amps especially. Parallel groups shouldn’t even be legal in modern battery architecture, because it does make unstable cells a hidden fire danger. They let people continue using cells that are finished, but hidden in parallel groups.

It is not more complicated to parallel serial strings than to series parallel groups. Only it can’t use common, cheap methods of assembly and monitoring.

In fact first parallel then serial is technically much better

the fact that many here spot weld the cells to build packs is what favours multiple strings "sub packs" paralleled at the pack level voltage.

If there were good no-weld technologies for bouncing around off road, then you could get the best of both worlds.

Actually best would be each cell having the full desired Ah capacity so 1P, but again, just difficult to do without space for a big boxy pack.