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I am building a "smart" battery by combining a bunch of Samsung 18650 cells with a battery management board that handles charging/discharging/protection.

The enclosure I plan to use allows for 4 x 7 cells, and I need 12-14.4 V range (this will replace a lead acid battery).

The architecture I want to use is thus: 3.6V x 4S x 7P (3.6V x 4S = 14.4V).

So the question is, I found a battery management board that supports 4S (here). Can I safely charge 7 cells in parallel? and what other options do I have?

Thanks!

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When you say you need a range of 12-14.4V, 4s li-ion cells will not fit in that voltage range. They are about 4.2V when fully charged -> almsot 17v. So if whatever this is powering can't take that then that is the first problem. It will also droop below 12v possibly depending on how you use it.

So, can you charge 7s in parallel. Of course. 7p is common in electric bicycle battery packs, they are charged in parallel. They are also bulk charged generally (though it is fine to individually charge the groupings as well. This is your other option.)

Bulk charged means you charge the battery as though it was just one big battery cell now and you dont give any heed to their being a bunch of batteries in series and parallel anymore. And individual charging would mean you charge each parallel grouping by its self.

In bulk charging however you need to monitor the individual groups voltages (overcharging these cells is not allowed (leads to fire ) This is why, when bulk charging, you must have a BMS unit to open circuit the charge path and to also balance the cells (not all BMS units balance. Even of the ones that do, most of them do a pretty bad job of it.)

If you want to individually charge them you can buy a charger that will take the bulk leads as well as individual leads of each of the electrodes of the 4 groupings and take care of charing them evenly. It is annoying a bit, but that is your decision to make.

By the way, many cheap BMS units go bad and parasitically drain your cells all the way to zero. The cells are supposed to stop at 3v so this is bad news for the pack. I used bluetooth connecting BMS units so I can monitor the states of the cells and be sure that all is well. They are more pricey though. With higher price you (hopefully) get better balancing functions though.

Oh, of course you will have to charge it with the correct charger (a CC-CV charger is what you need) and you will want to be sure not to charge it too quickly. The cells will have a charge rate on the datasheet) maybe around 1.5amps. 7x that number is fine for your application.

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To make a 7P battery, you need an entire Li-ion battery factory, because cells must be carefully matched in terms of internal resistance. If you are using a single charger with 7X current (to match 7P cells) and if the cells are not matched, some cells will be overcharged (overexposed to peak voltage beyond cut-off current), and some under-charged. On discharge cycle similar disparity will occur, with uneven wear of cells with smallest ESR. The 7P battery sections will have reduced life time and won't meet the expected capacity.

So you either need a whole factory, or use a fairly sophisticated battery management system with individual cell balanced chargers. You can make 7 systems to manage 7 chains of 4S batteries, and then a BMS for 7 parallel chains, something like that.

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  • \$\begingroup\$ Cells charged in parallel will always be charged to the same voltage. If the internal resistance is different, it will take different amounts of time, and the current through each cell will be different, but the end voltage will be identical. \$\endgroup\$ – Dampmaskin Sep 4 '17 at 11:56
  • \$\begingroup\$ *phrew, I was really questioning myself after this answer! \$\endgroup\$ – Lennart Rolland Sep 4 '17 at 12:25
  • \$\begingroup\$ @Dampmaskin, please re-read carefully what I wrote: "overexposed to peak voltage beyond cut-off current". I was specifically careful with this formulation. At no moment it implies "different voltage". It is known that Li-Ion cells don't tolerate long exposure to peak voltage, some solid-state chemistry incurs irreversible change, batteryuniversity.com/learn/article/… Please retract your unfounded comment. \$\endgroup\$ – Ale..chenski Sep 4 '17 at 16:15
  • \$\begingroup\$ A properly designed Lithium-ion charger goes into constant voltage mode when the voltage approaches peak. The charger in the OP's link cuts charging when it sees 4.2 +/- 0.05V and doesn't start charging again until it sees 4.05V, according to the specifications listed. Please explain how this overexposes parallel cells to peak voltage. \$\endgroup\$ – Dampmaskin Sep 4 '17 at 16:44
  • \$\begingroup\$ According to your Battery University link, degradation occurs when cells are kept at "a voltage above 4.10V/cell at elevated temperature". Granted, I can see how keeping them above 4.10V/cell for a long time is less than ideal, but I don't see why that would be the case for the constant voltage phase of a charger that is able to supply up to 30A. I'm not aware of any 18650 that should be charged as quickly as 4.2A anyway, so the charger in question seems overpowered if anything. \$\endgroup\$ – Dampmaskin Sep 4 '17 at 17:02

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