running battery without BMS given incomplete charge/discharge

fors17 · Sep 11, 2018

Hello,
I wanted to have your opinion on a simplified battery design. I have 48V system. I built 12s lipo battery, 15Ah (from large multistar battery packs from Hobbyking). I did on purpose 12s instead of 13s, so that at low voltage cutoff each cell has an average voltage about 3.6V, which is about 20% of capacity. I am charging it with 50.4V low current (1.5A) charger, connected in series with 3 diodes so that output voltage is 49V i.e. 4.1V per cells, i.e. ~80% of capacity. I'm not using BMS with the idea that running battery between 20-80% will allow disbalanced cells to avoid overcharge/overdischarge. Once a week or so I will be charging battery with balance-charger. Besides, running battery between 20% and 80% will help its longevity at the price of capacity, which I am OK with.

Thank you in advance!

dustNbone · Sep 11, 2018

It should be fine, there are people here that do basically the same thing. I'd recommend keeping celllogs or something similar that displays the cell voltages attached so you can have a quick look after charge/discharge to be sure there's nothing terrible happening.

MadRhino · Sep 11, 2018

RC lipo has strict rules of management. You don’t need a BMS, you must be constant with the rules.

The first rule:
Never let RC lipo charging out of your immediate presence, unless they are in a 100% fireproof enclosed area.

The 2nd rule:
RC lipo must sleep about storage voltage and unplugged off anything. This is the rule that half of RC lipo users are quick to forget, those using a BMS especially.

knurf · Sep 15, 2018

The second rule, is this simply to prevent slow draining of the cells after months of no use or do you see other problems with having it on a bms more permanently?

SirLongAss · Sep 15, 2018

MadRhino said:
RC lipo has strict rules of management. You don’t need a BMS, you must be constant with the rules.

The first rule:
Never let RC lipo charging out of your immediate presence, unless they are in a 100% fireproof enclosed area.

The 2nd rule:
RC lipo must sleep about storage voltage and unplugged off anything. This is the rule that half of RC lipo users are quick to forget, those using a BMS especially.

This.

fors17 said:
Hello,
I wanted to have your opinion on a simplified battery design. I have 48V system. I built 12s lipo battery, 15Ah (from large multistar battery packs from Hobbyking). I did on purpose 12s instead of 13s, so that at low voltage cutoff each cell has an average voltage about 3.6V, which is about 20% of capacity. I am charging it with 50.4V low current (1.5A) charger, connected in series with 3 diodes so that output voltage is 49V i.e. 4.1V per cells, i.e. ~80% of capacity. I'm not using BMS with the idea that running battery between 20-80% will allow disbalanced cells to avoid overcharge/overdischarge. Once a week or so I will be charging battery with balance-charger. Besides, running battery between 20% and 80% will help its longevity at the price of capacity, which I am OK with.

Thank you in advance!

I ran the same setup on my winter ebike. However, I had a lipo charger that could charge both batteries at 4A each. If you aren't interested in getting a proper lipo charger just get two of those lipo cell checkers. $4 each.
https://hobbyking.com/en_us/cell-checker-with-low-voltage-alarm-2s-8s.html

Or even better a cell balancer. $12 each.
https://hobbyking.com/en_us/turnigy-dlux-lipo-battery-cell-display-and-balancer-2s-6s.html

fors17 · Sep 16, 2018

The last balancer is exactly what i have. I have a couple of those and they work fine. I was thinking to keep them constantly connected, but they fell asleep after balancing is done.
What to you guys think would be the optimal voltage for unbalanced charging? 48 or 49 v? The last thing i want is to overcharge week cell to 4.4v and have it blown up.

MadRhino · Sep 16, 2018

Charging high doesn’t affect balance, but it can be done only just before riding. 4.4v is dangerous, don’t go that high unless you are there monitoring temp and you don’t care about battery lifespan. It is discharging below 3.7v that does affect balance. When you do discharge too low, you have to balance charge immediately, as soon as they are cool, to bring them back matching together at full charge. As long as you never discharge them low, RC lipo can go very long staying pretty well balanced.

running battery without BMS given incomplete charge/discharge

fors17

1 µW

dustNbone

10 kW

MadRhino

100 GW

knurf

100 W

SirLongAss

100 mW

fors17

1 µW

MadRhino

100 GW

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