Monitor individual cell health for your battery pack/low voltage alarm

[robca]

Thanks to an idea in the BMS/shunting thread (http://forum.electricunicycle.org/topic/459-electric-unicycles-bms-problem-and-solution/), I'd like to suggest this mod to monitor the individual cell health of a battery pack.

Assuming you don't mind soldering, it's a cheap (~$6 plus a couple of weeks waiting time for shipping) and effective solution that also adds a single cell alarm to warn you of possible ageing problems ahead of time.

I use these for my RC models, and they work exceptionally well: http://www.ebay.com/itm/FC-1-8S-Lipo-Li-ion-Fe-Battery-Voltage-2IN1-Tester-Low-Voltage-Buzzer-Alarm-/221849272720 (if the link doesn't work, search for "lipo alarm 8s", picture below). The alarm can be set between 2.7 and 3.9, and the alarm is super-loud. Each cell is monitored independently (i.e. if you set the alarm for 2.8V, as soon as just one cell hits that value, it starts beeping). You will need 2 of them (since one monitors up to 8 cells)

Then 2 of these http://www.ebay.com/itm/New-8S-JST-XH-Connector-balance-silicone-wire-CellLog-9-PIN-/181713383731 (if the link doesn't work, search for "jst 8s", picture below). They have nice, super flexible silicone wires, that will simplify positioning things inside the limited space.

Finally solder those wires to your battery pack. That's the past where you need to locate each battery terminal, cut a small hole in the heat shrink, and solder a wire where battery meet, following the enclosed schema (see below). There is a point where the GND of one alarm is connected to the 8s terminal of the other (since that alarm is designed to measure a series of batteries). Pay attention to the red wire on each connector (marked on the circuit), and solder wires in order. Insert the alarms into the JST connectors (respecting polarity, red wire to 8s terminal, but they are protected in case you insert them upside down). The alarms will show the exact value of each cell in turn, then the value of their half pack (should show between 25 and 33.6V).

If you set the alarm to, say, 2.8-3V, you will get a sharp beep as soon as one of the cells goes that low (with no cutoff or other side effects, if you shunted). If you place the alarms so you can see them, it will tell you which cell is the lowest. If you see that one cell is always the one triggering the alarm, you know you have a cell that is at risk. If different cells trigger the alarm, you have a healthy pack

While in theory this could be replicated for bigger packs (32 and 64 cells), given that those packs are used in parallel, the risk of a overloaded battery is much lower. This seems a good insurance for single pack EUCs

Please note that the alarm draws 13 mA when connected, so you should either add a switch on the GND leg to disconnect it when storing the EUC for a long time (during short trips the impact will be negligible) or remove the alarm from the JST connector when storing it. It will take a super-long time to actually discharge a battery, but I think that its power is always provided by the cell connected between GND and pin 1, so that battery will end up being over-discharged compared to the rest of the pack if you leave it on for more than a week)

[Chriull]

There is no standby/operating current in the description of the Battery Tester/Alarm. Is the display on all the time, or just when the Alarm is triggered?

Cell Voltage logging could be a nice addition to the http://forum.electricunicycle.org/topic/753-currentvoltage-measurement-and-heat-dissipation/?page=1 project?

[robca]

The display is on all the time, and first it shows the number of cells detected, then the total voltage, then all the individual cells one after another, and repeats forever. It uses 13mA when active, so probably a good idea to disconnect it when not in use for a long time (you can add a switch on the GND leg, it will completely disable it). If you put the JST in an easily accessible place, it can be easily inserted/removed (that's how they are used in RC models: insert before flying, remove as soon as you land)

Didn't think of the long term power consumption when the EUC is stored, good catch

[Rotator]

Thanks to an idea in the BMS/shunting thread (http://forum.electricunicycle.org/topic/459-electric-unicycles-bms-problem-and-solution/), I'd like to suggest this mod to monitor the individual cell health of a battery pack.

Great idea!

I know this little devices since I have RC-models as well. The mine ones are not very accurate, but I think that they will do the job quite well.

Easy and cheap solution.

For those people that don't want to solder wires into the battery, may be a total voltage alarm (40V?) in the battery output connector will be enough, instead of the cell-by-cell one. 

[robca]

I have more than half a dozen of them, and only 1 is what I would consider "off" when measuring. All the others are within 2% of a good multimeter. The best part is that if they are off, they are consistently off (say, 10% higher), so you can "mentally calibrate" if you get a bad one (and set the alarm accordingly). But you are right and, granted, I would not consider them "high quality voltmeters" by any stretch of imagination . But they are cheap and, at least the model I pointed to, very loud (there are other models, but not all are as good, or go up to 8S)

The EUC is already monitoring the total voltage. The risk (especially when you shunt a single battery pack), is that when drawing a lot of current (at or above the battery specs), a weak cell could end up dropping below a safe voltage. In that case, the rest of the pack will still provide enough voltage not to signal a problem, but you will keep stressing and overloading that same battery.Each time it's stressed, its chemistry deteriorates further, increasing internal resistance, and making it more likely that the same cell will be overloaded and over heated more easily than others. After a few cycles, you can have a dramatic failure of a single battery when the rest of the pack is still doing fine. Never a battery pack goes in flame as a whole: it's always a single battery failing and triggering a chain reaction. Replacing a single marginal battery is cheaper than an entire pack (not to mention the risk of physical damage to the rider and EUC when a cell fails)

This becomes more and more critical the longer a pack has been used/recharged, since chemistry aging will never be the same for every single cell in a pack. Monitoring each cell become more and more important as the pack nears its life limit

 

[Maxime]

I have a Charge Doctor which shows the bat voltage all along the charge and especially when finished. As long as this one will not change, I think my batteries stay good. Don't measure after a long time because batteries equalise slowly even when not charging and the mean voltage can be "not so bad" even if one is too low.