Battery protection in series

[brindle]

In many of my projects I use 18650 batteries with a protection circuit. For examle the DW01A or something similar like the protection IC's from RICOH.

However, I would like to use some batteries in series but I find it very hard to find something similar for cells in series. I figured these things basically do an 'internal' comparison, and that it may not matter if they are connected to +4v/GND and +8v/+4v since it would look all the same to the IC.

The use would basically look like the image attached.

Would this work as I would expect? That is, I want to prevent the batteries from discharging too far.

[tunk]

You could try to search for this on ebay or aliexpress: BMS 2S

[tszaboo]

When one of your circuit switches off, the FET's should become an open circuit. But it doesn't actually does that, there is a tiny bit of current still going. With the other battery still conducting, you get voltages connected to pins that are not designed to do that. So my suggestion is to use a dual BMS circuit.

[robert.rozee]

hi brindle,
    what you have shown in your schematic is exactly what happens when two or more 'protected' cells are connected in series, and you can purchase small PCBs containing a DW01A and a pair of MOSFETs ready to wire across individual 'unprotected' cells.

where things get complicated is when you are charging 2 or more 'protected' cells in series, and one reaches full capacity before the other(s). there is the possibility of the DW01A of the full cell opening the over-charge MOSFET for that cell, which will then prevent further charging of any of the other cells in series.

in an ideal world you would therefore have charging connections for each individual cell (so for a string of N cells there would be N+1 connections) and have N floating chargers, one for each cell.

the BMS for multi-cell packs may get around the problem by having shunt transistors across each cell that start bypassing current when a given cell reaches the 'full charge' voltage. but i have never really liked this solution as (a) it wastes power, (b) it may interfere with detecting the current drop-off at end of charge, and (c) most implementations i have seen seem to be rather badly designed.

for one such balancer design, see here:
https://320volt.com/en/simple-balancer-circuit-for-li-ion-batteries/

but note that this circuit will  continue to draw current from cells (through R1+R2) after the charging current is removed. really this sort of circuit needs to be isolated from the cell when not charging - and indeed, when equipment is turned off there is no need for any of the battery protection or management circuitry to be connected.


cheers,
rob   :-)

[Peabody]

For over-discharge protection, I think you have to have protection for each cell so the circuit will shut down when any cell falls below the over-discharge voltage limit.  And it looks like your circuit would work.

Charging presents different issues, including the need to balance the cells.  I think that's why many prefer to use a single cell, or multiple cells in parallel, and a boost converter.

But note that you can buy protected 18650s which would save on the parts count.  But you'd still have the charging issues.  And you wouldn't use those in parallel.

[brindle]

You could try to search for this on ebay or aliexpress: BMS 2S

I like to design the circuit in easyeda and have it all neatly on a pcb togheter with whatever I use it with (an ESP32 for example). I did look on Aliexpress to see if I could find anything on the IC's used on those boards but not much luck.

Putting these 1 cell protection circuits in series doesn't sound like a good (or safe) idea, so I will just use single cells and a booster. I see a handful of things on the webpage of TI where they have IC's that do 2 cells and some magic with a power path, but those are way too complicated for what I am doing unfortunately.

I figure I might as well make my own battery protection. I can monitor battery voltage with the ESP and put the entire unit to sleep when it goes below a certain voltage. As a backup I can use a comparator to cut off the voltage to my device. I think I found a nice one with a low power consumption: TLV6713, see picture attached. I don't quite understand the datasheet though:

in 3. it says the following:
OUT is driven low when the voltage at the SENSE
pin drops below the negative threshold, and goes
high when the voltage returns above the positive
threshold

in 8.3.1. it says this:
For the comparator, the output (OUT) is driven to logic low when the input SENSE voltage drops below VIT–.
When the voltage exceeds VIT+, OUT goes to a high-impedance state

This seems contradictive to me, how do I read this? When can the OUT pin sink current, when my SENSE voltage is above the internal reference or below it?

[Peabody]

The first statement describes the behavior of the part as a whole.  The second is talking about the comparator inside the chip, which is followed by an inverter.  The bottom line is when the voltage drops, the output goes low.

I think it makes sense to monitor the battery voltage with your processor, and shut things down well ahead of when the DW01 would do that.   But I'm not sure putting things to sleep is enough to stop battery drain.   You may also need to shut down the boost converter.  It will continue to draw current even if it has no load.

[brindle]

That is true, it will continue to drain at a certain rate but that is not a problem. I would have the processor cut it out at 6V for 2 cells, leaving enough time to wait for some good weather to go into the garden and charge the batteries before their voltage gets too low.