Advice needed re Small 18650 pack

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Phester

New member
Joined
Nov 2, 2021
Messages
3
HI All,

I am building a small 7p7s pack from dyson battery cells, all are high discharge cells . I am currently profiling their capacities. V droop and recovery voltage. some are as low as 1300mah at 1A discharge to 3V more are 1600-1900mah. Or i may just do a 6s8p as I have a Imax B6 clone charger and it will balance 6s pack. I also have some laptop batteries but so far they are testing very low capacities and all would be low C ratings. I dont have any access to more cells so I will have to proceed with the best of the bad lot. I also will be able to measure AC impedance of each cell to best match the Z and capacities

I have searched everywhere for an answer on this as my use for the pack is very different to most.

I want a 24V pack to be kept topped up from my solar panel (it is small) only a 10w (10v 2A) I have a boost buck with CC/CV so I should be able to supply the pack voltage required to charge at a low current rating. It will not be drained typically (no load) as I only want to use it if we have a power outage on my 24V inverter. (Typically 70W) so about 3A from each cell group or roughly 0.5A per cell


Most of my cells seem to fall to 4.1 or so volts shortly after a full charge so I would expect to apply somewhere in the region of 4.05 or so volts per cell group to reduce degradation and limit imbalance risk of a high voltage on one cell. All cells stay at 4.0x volts after 3 or more months

Is a cheap BMS a waste here as it will only be a drain on one cell. Balance function limit would never be hit as I am charging below the preset threshold. Only considering cheap aliexpress BMS . In the winter I would have a low level of light so winter charging (topping up) is going to be limited. Grey weather
 
Last edited:

OffGridInTheCity

Active member
Joined
Dec 15, 2018
Messages
1,785
HI All,

I am building a small 7p7s pack from dyson battery cells, all are high discharge cells . I am currently profiling their capacities. V droop and recovery voltage. some are as low as 1300mah at 1A discharge to 3V more are 1600-1900mah. Or i may just do a 6s8p as I have a Imax B6 clone charger and it will balance 6s pack. I also have some laptop batteries but so far they are testing very low capacities and all would be low C ratings. I dont have any access to more cells so I will have to proceed with the best of the bad lot. I also will be able to measure AC impedance of each cell to best match the Z and capacities

I have searched everywhere for an answer on this as my use for the pack is very different to most.

I want a 24V pack to be kept topped up from my solar panel (it is small) only a 10w (10v 2A) I have a boost buck with CC/CV so I should be able to supply the pack voltage required to charge at a low current rating. It will not be drained typically (no load) as I only want to use it if we have a power outage on my 24V inverter. (Typically 70W) so about 3A from each cell group or roughly 0.5A per cell
You comments make sense. You might want to discard the 1300mah cell and stick with 1600-1900mah cells for a battery if you can. If you can't, evenly distribute the 1300mah cells among the packs - in fact you want to evenly distribute the cells by capacity thru the packs so all the packs in series have a similar ah / make-up as you can.

Most of my cells seem to fall to 4.1 or so volts shortly after a full charge so I would expect to apply somewhere in the region of 4.05 or so volts per cell group to reduce degradation and limit imbalance risk of a high voltage on one cell. All cells stay at 4.0x volts after 3 or more months
After full 4.2v charge - cells will almost immediately fall to 4.15v. Over several weeks they should still be 4.10v or higher else you have self-discharging cells and you should discard those as you won't be able to keep a 7s or 6s battery in balance.

Is a cheap BMS a waste here as it will only be a drain on one cell. Balance function limit would never be hit as I am charging below the preset threshold. Only
considering cheap aliexpress BMS . In the winter I would have a low level of light so winter charging (topping up) is going to be limited. Grey weather
A BMS is *strongly* recommended! Both for balance but more importantly for min/max voltage with cut-off for each pack in series. Even if you top out at 4.0v/pack (4 * 7 = 28v), bad pack(s) going lower can push up other packs as the battery maintains the fixed 28v setting and could eventually push a pack over 4.2v - which can lead to fire.

Believe it or not a battery can even appear to run with a pack going into negative voltage and the others (in series) going higher to compensate. This is where the min of the BMS can save things as well. A pack forced into negative voltage can be dangerous as well.
 

Phester

New member
Joined
Nov 2, 2021
Messages
3
thanks for the explanations: I understand them all.

One query is if the pack is never cycled, am I better to have the bms disconnected and only connect up both charge (solar) and bms for a top up every few months, or just let it get a slow trickle from the panel boost regulator with BMS online permanently. For the longevity of the pack

Do the Chinese BMS disconnect the O/P indefinitely when a cell group is below the cut off (until voltage issue rectified)

I can also do a CV CC from a variable power supply if needed or use the Imax B6 but only up to the 6s
 

Oberfail

Member
Joined
Jun 22, 2021
Messages
103
Here is a tool to put the cells into good groups: https://www.repackr.com/#/pack-builder
Just put in the raw capacity numbers, press enter and select how you wanna group them (7s7p for example) and it will output you the ideal configuration.

As already mentioned, its normal that the cells voltage drops directly after charging to 4.15ish volts or slightly lower. Its also recommended to not charge them beyond 4.1 volts as that helps with longjevity of those cells.

You'd have to test the BMS yourself, but most dont disconnect completely and still have a slight parasitic load. Just check every once in a while, how the batteries are holding up or get a fancy BMS. Those give you controls over certain limits like the charge voltage and disconnect voltage. So you can set it to only charge to 3.6v while you store them and use them slightly and set the limit to 4.2 (or better just 4.1) if you need the battery.
 

OffGridInTheCity

Active member
Joined
Dec 15, 2018
Messages
1,785
In the first post you said "...I want a 24V pack to be kept topped up from my solar panel (it is small)...." and so I'm picturing you have panels / the batteries would be online all the time and just 'topped off a bit' each day as you get some PV power.

One query is if the pack is never cycled, am I better to have the bms disconnected and only connect up both charge (solar) and bms for a top up every few months, or just let it get a slow trickle from the panel boost regulator with BMS online permanently. For the longevity of the pack
I'm picturing connected all the time? If not, then you can put in an on/off switch - which could disconnect the battery and BMS at the same time from the rest of the system. When you say "Dyson" I'm thinking 18650(s) from Dyson battery packs... lithium-ion 18650 cells can sit months and retain the voltage they have at 'off time'. So turning everything off for long periods should be OK.

Do the Chinese BMS disconnect the O/P indefinitely when a cell group is below the cut off (until voltage issue rectified)
I can also do a CV CC from a variable power supply if needed or use the Imax B6 but only up to the 6s
If you have a BMS go low and cut-off due to low pack you can always bypass the BMS and do a little direct charging and then reset the BMS. There are several flavors of BMSs and you can make sure it includes specifics or pick something well know like Daly where there's online specs.

What I do is have that OFF switch... that disconnects both the battery and the BMS for long term storage. Others may have different approaches and may comment :)
 
Last edited:

Phester

New member
Joined
Nov 2, 2021
Messages
3
Correct Dyson battery 18650 from v6 and v7. Solar just tops off the pack. Its a small panel only 10w at 10v so I'll be using a Chinese boost buck. I may have to put a diode on the O/P of same and increase voltage accordingly (0.6v)

I've ordered the following bms with balance leads.
I doubt it will do 25A but I am running much less around 3A continuous but will test at 10A and use a flir camera to check for heaters. Picked this over other similar BMS as it says it will cut of at 2.8v other budget bms were 2.5V which is too low for my liking. I will use my crap cells to make a dummy 24 volt pack first and check its features work.
https://www.aliexpress.com/item/1005003178052797.html?spm=a2g0s.12269583.0.0.7bd74d66uca718

Volt meter with separate supply: So i can power from panel and display pack voltage
https://www.aliexpress.com/item/32953103514.html?spm=a2g0s.12269583.0.0.45c834afMyZoJv

And one of these cell voltage meters with alarm so I can quickly verify pack balance Bx100
https://www.aliexpress.com/item/400...36-0&pdp_ext_f={"sku_id":"10000000108419721"}
 
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