Temperature Monitoring with a BMS

Hi all,

BMS-related question...

So I've been working on the battery design for my first ebike, I want to make a DIY design mainly for fun as a hobby and to get some hands-on experience with batteries to learn more about them.

My current idea is to create two, 2P10S battery modules (constructed from 18650 cells) and connecting them in parallel to essentially create a 4P10S battery. Each module will have its own BMS and will be charged using a separate charger.
Motor is a Bafang BBS01B, 250W, limited to 15A (I'm not going for a monster bike haha).

One feature I'd really like to incorporate into each module is a temperature sensor that has an input into the BMS. If the temperature exceeds 'X' oC (haven't determined a value), the BMS will shut off the electrical circuit.

I've spent several hours browsing previous forums for advice but I'm not sure I'm any wiser on the topic!

I've tried bullet pointing my thoughts and concerns out below.

1. Finding appropriate BMS.

I'm not keen on the made in China products that dominate the ebike BMS market, especially considering this is a safety critical device (if I'm not manually monitoring my cells). I would be open to paying more for a product made either in the USA or closer to my home in the UK, or even Europe if there is proof that the BMS will work how it says it will - if there's decent technical support available should an issue arise, then even better!
I've seen the Energus TinyBMS from Lithuania which I thought looked like a much more trustworthy source built with decent quality electrical components. The fact it has an input port for temperature measurements was very appealing. However, I have seen on a few forums that the product is quite 'buggy' and the aftersales support is terrible.
Alternatives such as the link below are better priced with all the standard features I'd expect to see on a BMS - plus, its from a non-Chinese retailer that gives me a bit more hope. But... No input for the temperature sensor.
So my first question is, can anybody recommend any 10S BMS's, ideally non-China models, at least capable of 15A continuous current, ideally having a temperature input port for thermal monitoring.

https://vruzend.co.uk/collections/10s/products/10s-36v-15a-lithium-battery-bms-balance-function-18650-li-ion-uk-seller-stock

2. Alternative solutions.

One alternative solution would be to use an Arduino board to monitor the temperature within the battery (it could even monitor several temperatures). If the temperature reading exceeded a certain amount, then the Arduino could open a relay that closes the electrical circuit between the module and motor controller.
Does this sound like a sensible idea/can anyone see any problems with doing this? If anyone has any experience with this sort of thing then I'd love to hear more.

3. Issues with implementing an 'over-temperature-shut-down thing'.

A concern I do have with this feature is that as I'm paralleling two batteries, if one batteries overheats and shuts down, the other one could in theory continue running, providing the full 15A that the motor controller demands. This is not good, as in an ideal situation, I want to keep the state of charge of each pack as close together as possible to prevent reverse current flow when reconnecting the packs. How could I simultaneously disconnect both batteries and kill power to the motor controller? (most obviously having a relay/contactor before entering the motor controller).

Thanks in advice, any knowledge and advice would be very welcome and extremely appreciated as I am currently unsure how to proceed!

Thanks,

N.

If you can live with made-in-china, get one of those programmable jbd bms. The software is super buggy, but it works ... and comes with 2 temp sensors! Also, they have a switch pin. If you connect them in parallel, you can switch off both batteries at the same time.

Hi qwerkus, thanks for the reply!

May I ask what sort of software bugs you've experienced with the JBD BMS's? I'm just a bit concerned that a bug could cause the BMS to shut off mid ride or worse. I'm quite a cautious person so I need to have faith that the BMS isn't going to die, leading to a big fire or damaged battery.

Also, my apologies if I'm coming off a bit thick here (it happens a lot with me on an alarmingly regular basis), but would you not want the switch pins connected in series? If one switch goes out then the whole battery system goes off, no?

Nothing serious, just glitches. There is a huge thread here dealing with the jbd boards and the software. The bms won't let you down, but sometimes the phone app seem to lose connection or crash. I don't use it so I can't confirm.

Here is a picture of the configuration page of the pc software, you can pretty much change everything on the battery:


Most useful by far I find the possibility to limit charge to 90% and still have balancing to increase cell lifetime. I usually set the charge limit to 4.05V per cell on huge packs and 4.10-4.15 on small packs.

If you want to monitor two batteries, you need indeed 2 phones or 1 hacked phone with 2 BT cards as I don't think BT can be shared. Or you write your custom app with switches batteries connection every 1s. I wouldn't recommend it though. A simple way to monitor batteries is a cheap chinese volt/amp meter. Sure, you'd need wires but its a simple an sturdy solution.

In my experience temperature monitoring is kind of useless. If you use the jbd bms, you set a max working temp (50-60°C for the cells and 80-90°C for the fets) and forget about it. The battery will automatically shut off when reaching that threshold. Of course you need to route the sensors to relevant places, like 1 between the cell in the middle of the pack, and one next to the power mosfets.

Thanks for getting back to me qwerkus, I'll take a closer look at the thread now.

The configurability looks like a big plus. Cheers for sharing the screenshot!

Yeah I'm going to scrap the 2 phone option, seems completely pointless really. Instead I'm going to follow your advice and keep it simple with a current/voltage/wattmeter.

Also agree with the temperature stuff.

Thank you for clearing all that stuff up, I really appreciate it.

NiCat23 said:

Yeah I'm going to scrap the 2 phone option, seems completely pointless really. Instead I'm going to follow your advice and keep it simple with a current/voltage/wattmeter.

Click to expand...

Don't forget to calibrate them before use! There is usually a small potentiometer to do it inside.