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© 2017 Ian Watts

Pack Isolation, Protection & Charging

Pack Isolation and Protection

Simplistically, my selected BMS should protect my battery packs by keeping them within their safe operating 'zones'. It should provide (at the minimum) :

1: over-voltage (charge) protection (> 4V2 per paralleled cell pack),

2: under-voltage (discharge) protection (<2V8 per paralleled cell-pack),

3: balancing (charge) feature and

4: over-current (discharge) protection

A quality BMS may also protect the pack against over-temperature and over-current (charge. It may also report battery health and status information to one or more (remote) displays.

A BMS will not, however, provide an isolation switch or an easy means of connecting / disconnecting the pack to / from its intended 'load' (eg: motor / controller) or to its charger. Similarly, whilst the BMS should shut down the output on over-current this will be the over-current value selected when purchasing the BMS itself. Switching out the BMS to suit a specific application is not a simple task (see the BMS section) and whilst one application for a 72v (nom) battery may be a 3KW electric scooter that requires up to 40+ amps, a 1.5KW lawnmower, using the same battery, may only require somewhere in the region of 20A.

In theory the selected generic 60A BMS should isolate the battery from its circuit when it detects a current draw of (circa) 60A or more for a specific period of time (see the spec. sheet when selecting your BMS). In each of the cases above, we could specify a 40A BMS or a 20A BMS to protect the load / battery pack / cabling for each specific application... Or we could modify the circuit's cabling and in-line fusing to suit...

Excerpt (1) from typical BMS Specification sheet :

  • 3S 12.6V 60A lithium battery protection board (comes with recovery function-AUTO Recovery)

  • Type: Balanced Version

  • Application: Nominal voltage of 3.6V, 3.7V lithium battery (including 18650, 26650, polymer lithium battery)

  • Continuous discharge current (upper limit): 60A (if the cooling environment is not good, please reduce the load current use)

  • Continuous charge current (upper limit): 20A

  • Charging voltage: 12.6V - 13.6V

  • Size: about 4.2x6cm/1.65x2.36"

Excerpt (2) from typical BMS Specification sheet :

  • Type of batteries : lithium cobalt / manganese lithium / ternary materials

  • Single overcharge protection voltage 4.25V ± 0.05V (4.20-4.35V / / 0.05V per upgrade )

  • Single overcharge recovery voltage :4.10-4 .00 V

  • Single Over-discharge protection voltage : 2.50V ± 0.1V (2.50-3.0V / / every .05 V into the class )

  • Single Over-discharge recovery voltage : 2.80V ± 0.1V

  • Protection Current consumption : ≤ 300UA

  • Short circuit protection current : 40A ± 3A

  • Short circuit protection time : 500MS

  • Temperature protection : 55 / 65 / 75 degrees

  • Discharge Current : 30A

  • The maximum instantaneous current : 40A

  • Single balanced voltage : 4.19V ± 0.02V

  • Single balanced current : ≤ 55MA

  • Charging current : ≤ 10A

To simplify the connection and specific-to-applictaion current trip 'issues', outlined above, I prefer to add a suitable thermal circuit breaker to each battery pack. Such a circuit breaker provides a simple method of connecting both the battery and the load / charge interconnect cables whilst also providing a 'switch' to totally isolate the battery when needed.

The battery pack is hard-wired (internally to the pack) to the thermal circuit breaker and the output of the breaker is terminated with a suitable high current plug / socket connecter, such as an Anderson SB50.

50A 80V (Rated)

Thermal Circuit Breaker

Anderson SB50

50A Connector

Additional (to any BMS) Thermal Protection

I have some ideas re: temperature protection for my larger, external (scoot and lawn mower) battery packs. At night temperatures here can drop below freezing and during the day they can rise to well above 30'C / 86+ Fahrenheit and I worry about the continued health of my battery packs.

I had the idea, with my larger packs' to include a pair of thermistors within the centre of them / sandwiched between the cells and, as a future project, use them in conjunction with a small AVR project and some additional components to enable the AVR to sense when to heat or cool the pack.

20S4P pack

Blue/Yell, Blue/Pink wire pairs = Thermistors

Anyways my larger battery packs incorporate the 2 two thermistors (blue/yellow and blue/pink cable pairs in the adjacent image) which are waiting for me to connect them to (for example) overnight heating when the charger is connected and in-use cooling should higher than optimum temperatures being detected... Currently I'm still 'playing' with the whole idea but it's coming along in conjunction with my coulomb counter / fuel gauge functionality. It's the mechanical design that's bugging me at the moment. I'll get around to blogging it at some point !

Charging

Again, not quite as simple as receiving a pre-approved, standard charger that could arrive with a purchased battery pack. There are a couple of things to consider here :

  • BMS. What type of BMS does (or will) the project have ? A single port where charge and discharge occur on the same cable pair (pretty much ALWAYS the case if Regen is involved) ? or a split port where the charge cabling (usually charge -ve) is connected separately to the discharge cabling ? (See the section on BMS types.)

  • Charge rate. Charge rate is usually specified in terms of C, where C is the stated battery capacity in mAHr. In the case of the Panasonic NCR18650GA cells, C is stated as being 3350mAHr and the optimum charge rate is stated as being 0.7C. This is for a single cell. Connecting cells in series (like a 20S pack) does not change the C rating of that pack BUT connecting cells in parallel does ! So a 4P pack has a 4 x 0.7C optimum charge rate = 4 x 0.7 x 3350mAhr = 9380mAHr or a 10A charger. You'd be quite within your rights to use a 5A charger but beware charging your battery pack too quickly with, say, a 20A charger. Batteries heat up during the charge process, charging batteries fast causes them to heat too much and too much heat causes irreversible cell chemistry damage - not to mention the fact that heat causes cell expansion and eventual venting or... worse... However you look at it, rapid charging, above the manufacturer's 'safe' zone will cause premature battery failure (at best) or a lot worse... Take care, fast charging is great but comes at a cost.

Gx16 XLR Aviation

Plug and Socket

In the image above I'm showing a 4 way connector pair; 2 pins for charging and two for data / warning output once I've completed the AVR monitor, cooling & heating module.

I'm not convinced that Regen on a converted 50cc scooter would be worth the effort / cost mostly down to limited kinetic energy being available due to the low weight of the scoot. Of course, a car (being far heavier) would be a different story ! (and as always, I stand to be corrected - to be totally frank, controllers with Regen are most often noticeably more than those without - and one-off projects (to test viability etc) are usually undertaken on a tight budget - I can always update a controller at a later date if I really believe it likely to be useful / efficient in a specific application - I don't think my lawnmower conversions would not benefit ! )

Either way a method of connecting a mains / plug-in charger to a battery is required and the design will dictate how to go about doing that. If, for example, the battery is to be removed from the platform for charging then a separate charge socket may not be necessary, however if the battery is to remain in / on the platform then a separate charge socket will be required if one is not to disconnect the battery for the load and reconnect it to the charger (hardly 'ideal'). Additionally, if a split-port BMS has been employed then there is no choice; a separate charge connector will be required.

For my uses (small hand-tool-type kit, lawn-mower conversions and my electric scoot project) I tend to favour split-port BMS units and separate charge sockets where the charge cabling bypasses the main power (motor / controller) cabling and for this I typically use a 4 pin XLR / Aviation plug and socket arrangement.