How to add a BMS to lipo batteries?

[drsolly]

Two trios of 4s 20c 5ah Lipos costs £93 and gives you 10ah 12s, about 450 watt-hours. And they're lighter than any other battery. They need a balance charger, but that should last indefinitely, and if you charge the 4ses as 4ses (rather than try to charge them as 12s), a fairly cheap (£8) charger http://hobbyking.com/hobbyking/store/__26010__HobbyKing_E4_Balance_Charger_UK_Warehouse_.html is all you need. Although I like to use a higher spec charger.
http://hobbyking.com/hobbyking/store/uh_viewItem.asp?idproduct=28150

By comparison, the last lifepo4 battery I got was 10ah 7s, so was 260 watt-hours, and cost me £196.

 

[shemozzle999]

How much time does it take you to break down the pack, charge and reassemble?

Might be fun the first few charges!

One of my set ups:

Frog battery 24v, 15A approx £190 delivered complete with matched charger and no labour required to get it up and running, currently over 200 charges and still going strong.

Charge routine: Unplug from bike, plug into charger, charge and then replug onto bike.

Battery weight accounts for less than 2% of my bike/rider set up.

 

[drsolly]

Break down = unplug two connectors
Charge - plug in two connectors, then wait three or four hours.
Reassemble = unplug two connectors from charger, then plug in two connectors.

For me, bike weight is more important that bike+rider, because I use it for geocaching, and occasionally I have to lift it over obstacles. Being able to take the batteries off with one unplug and one take-pannier-off is very handy.

 

[shemozzle999]

Surely with your assembly method and using the charger you mentioned you have to unplug from power feed (1), remove blocks from your container, break the parallel connections first (2), break the serial connections (4), remove the monitor boards (4), charge each block separately, or use a parallel board, pushing it with a 40W charger, reassemble back into serial blocks (4), refit monitoring boards (4), double check serial block voltage for any voltage difference between serial blocks before connecting back in parallel (2), refit back into container then reconnect power feed (1).

I am sure you are fully capable of working with lipos and of the safety required, but from many of the posts on this site there are many who do not and for that reason I do not usually contribute to these threads.

Arbol raised a suggestion of fusing each parallel branch something I would highly recommend.

 

[Arbol]

shemozzle999, may I ask your opinion on fusing each parallel branch?

I ask this because when I spoke with em3ev's Paul, he told me his triangle packs have fuses at the balance wire level, but he has no other fuses (in particular, he has no main fuse (!)).

Theoretically and naively, it seems to make sense to have, at least, a fuse at the main level (controlling the overall series connection) and fuses at the parallel level.

However, em3ev does not do it that way, but he uses the (apparently to me) "accessory" balance wires to add fuses.

em3ev is a guy with a good reputation, and if he does it that way, there must be a reason. I do not understand the reason, and for this reason I feel uncomfortable.

May I ask your opinion on that subject, please?

 

[shemozzle999]

Using one of Drsolly's battery builds 2p, 2s( 3x4s) as an example:

If all the lipo packs are purchased at the same time they should be from the same manufacturing batch, but not guaranteed, then each series string will share the output current i.e. for a 2p - half from one string and half from the other then the serial fuses can be rated at just over half the full output current.

If there is a problem with imbalance between the series strings, for what ever reason, then one of these fuses will blow effectively disconnecting the parallel circuit.

One possible scenario would be if one of the serial string packs was inadvertently disconnected before removing the parallel harness then the higher voltage rated serial string would try to charge the lower one. Not likely, but possible.

 

[Arbol]

Yes, thanks, shemozzle999.

 

[drsolly]

Surely with your assembly method and using the charger you mentioned you have to unplug from power feed (1), remove blocks from your container, break the parallel connections first (2), .

I rarely use parallel connections.

I don't remove the blocks from the container. The container just lifts out of the pannier, with the batteries still inside.

break the serial connections (4), remove the monitor boards (4), charge each block separately, or use a parallel board, pushing it with a 40W charger, reassemble back into serial blocks (4), refit monitoring boards (4),
.

I use a parallel board. Plus, I have four chargers. I don't refit the monitoring boards, I do that when I connect the battery to the bike.

double check serial block voltage for any voltage difference between serial blocks before connecting back in parallel (2), refit back into container then reconnect power feed (1).

I am sure you are fully capable of working with lipos and of the safety required, but from many of the posts on this site there are many who do not and for that reason I do not usually contribute to these threads.

Arbol raised a suggestion of fusing each parallel branch something I would highly recommend.

I rarely use parallel connections.

I have my batteries (20C 4S 5AH hobbyking hardcase) taped together in pairs. Today, I put three pair in my pannier box, connected one pair to the bike (plug in two EC5s to driver cable, plug in two balance leads to monitors.

When the monitor starts to beep (or maybe before) I unplug the two EC5s and the balance leads, then plug into the next pair of batteries. It takes about a minute to do all that.

When I got home, I put the box with the batteries near my balance chargers, and plugged one charger into each battery pair (parallel charging the two 4ses).


I get extemely poor milage from my bike, because of how I'm riding. Today, I got about 9km from 160 watt-hours (5AH, 8s). That's because I'm not riding fast over tarmac, I'm riding fairly slowly over grass and mud, and stopping every few hundred meters. That's because I'm geocaching.

Because I get such poor mileage (and because I can't accurately predict how much milage I'll get from a battery because I can't predict the track conditions), it's good for me to be able to carry extra batteries. And because I occasionally need to lift over an obstacle, It's good that I can very quickly unplug the battery and remove the panniers, thereby lightening the bike for lifting.


I agree, what I do might be too complicated for some people. But I love the flexibility it gives me (I can carry anything from one pair giving me 8s 5AH up to seven pair if I'm going a very long way).

 

[shemozzle999]

Thank you for your reply that clearly explains your requirements and your method of battery usage. You have obviously found a safe and practical solution.
I also find going off track i.e. even over grassy areas does tend to greatly hinder the progress of my trusty steed. I should imagine that your geocaching will find you getting into quite tricky territory to achieve a find.

 

[Arbol]

When I got home, I put the box with the batteries near my balance chargers, and plugged one charger into each battery pair (parallel charging the two 4ses).

drsolly, II do not fully understand that explanation. Don't you need to unplug the lipo batteries from their series harness in order to charge them?

I am considering a series harness for two 6s 5Ah batteries, providing 44V. Since batteries have a HXT4mm connector output (as well as the JST-XH connector for the balance cables), I am considering an HXT4mm series harness such as:

http://www.hobbyking.com/hobbyking/store/__18603__HXT4mm_Battery_Harness_14AWG_for_2_Packs_in_Series.html?strSearch=series harness

I assume I would have to disconnect the two batteries from the harness in order to charge and balance them with an RC charger, right?

 

[drsolly]

Yes.

I use two 4s batteries in series for driving the motor, so it's 8s. For charging, I unplug that driving (series) harness, and charge the batteries in parallel using my charging (parallel) harness. I make the harnesses myself, it's just a simple soldering job. I use EC5 connectors (I have a 4mm connected to an EC5 for each battery); because I'm going to be doing a lot of plugging and unplugging, I prefer those. It also means that if they wear, it'll be cheap and easy to replace them.

Your case would be the same. Unplug from the series harness, and then you have two 6S batteries. Then use your balance charger (you can get a parallel charging board from Hobbyking) to charge them both.

If you find that 5AH isn't enough, then you simply buy a second pair of 6s 5AH from Hobbyking.

By the way, look at the cost of three 4s 20C 5AH Hardcase batteries, and compare it with the two 6ses that you're planning on. It might be somewhat cheaper, and 4s chargers can be quite a lot cheaper than 6s. But it does mean one extra battery to plug and unplug each time.

 

[cwah]

If you can avoid lipo, I'm advising to avoid to get them. I've been using many lipo pack, and I'm still using my last lipo pack. But they are designed for RC and not for ebike. So your pack will suffer many inherent issues:
- Many potential points of failure: HK is not famous for its lipo quality. It's known for his price. You'll have from time to time some wire that will move out from the pack. Especially the balancing wires.
- Rubbing batterie packs: because you'll parallel and serie many packs together, they'll have potential to rub between each other. One solution is to glue all of them. But it's not the best one.
- Spaghetti wiring: No matter how hard you try, you'll end up with spaghetti wiring because of the amount of wiring harness you'll have to do. The only way to avoid that is the remove all the individual cells from the pack.
- disastrous consequence when one of these thing happen to only one of your 40-50 cells: Puncture, overcharge, overdischarge, overheat.

Few years ago, lipo was the only available choice that's why so many of us have chosen to buy from RC shops. But now, battery sector has evolved. Lipo are neither the cheapest or the highest in energy density.

The only reason to still buy lipo would be if you need high discharge rate. Otherwise, the pain factor (wiring, daily precautions and protection) and risk factor do not worth it.

Get some branded 18650. They are way simpler to manage, they are lighter, they are safer and can even be cheaper.

 

[mountainsport]

If you can avoid lipo, I'm advising to avoid to get them. I've been using many lipo pack, and I'm still using my last lipo pack. But they are designed for RC and not for ebike. So your pack will suffer many inherent issues:
- Many potential points of failure: HK is not famous for its lipo quality. It's known for his price. You'll have from time to time some wire that will move out from the pack. Especially the balancing wires.
- Rubbing batterie packs: because you'll parallel and serie many packs together, they'll have potential to rub between each other. One solution is to glue all of them. But it's not the best one.
- Spaghetti wiring: No matter how hard you try, you'll end up with spaghetti wiring because of the amount of wiring harness you'll have to do. The only way to avoid that is the remove all the individual cells from the pack.
- disastrous consequence when one of these thing happen to only one of your 40-50 cells: Puncture, overcharge, overdischarge, overheat.

Few years ago, lipo was the only available choice that's why so many of us have chosen to buy from RC shops. But now, battery sector has evolved. Lipo are neither the cheapest or the highest in energy density.

The only reason to still buy lipo would be if you need high discharge rate. Otherwise, the pain factor (wiring, daily precautions and protection) and risk factor do not worth it.

Get some branded 18650. They are way simpler to manage, they are lighter, they are safer and can even be cheaper.

Well said cwah I totally agree with you on this, the only way you could get away with using Lipos is if you are a professional at handling and correctly knowing how to maintain them.

MS

 

[Arbol]

If you can avoid lipo, I'm advising to avoid to get them. I've been using many lipo pack, and I'm still using my last lipo pack. But they are designed for RC and not for ebike. So your pack will suffer many inherent issues:
- Many potential points of failure: HK is not famous for its lipo quality. It's known for his price. You'll have from time to time some wire that will move out from the pack. Especially the balancing wires.
- Rubbing batterie packs: because you'll parallel and serie many packs together, they'll have potential to rub between each other. One solution is to glue all of them. But it's not the best one.
- Spaghetti wiring: No matter how hard you try, you'll end up with spaghetti wiring because of the amount of wiring harness you'll have to do. The only way to avoid that is the remove all the individual cells from the pack.
- disastrous consequence when one of these thing happen to only one of your 40-50 cells: Puncture, overcharge, overdischarge, overheat.

Few years ago, lipo was the only available choice that's why so many of us have chosen to buy from RC shops. But now, battery sector has evolved. Lipo are neither the cheapest or the highest in energy density.

The only reason to still buy lipo would be if you need high discharge rate. Otherwise, the pain factor (wiring, daily precautions and protection) and risk factor do not worth it.

Get some branded 18650. They are way simpler to manage, they are lighter, they are safer and can even be cheaper.

Interesting argumentation, cwah, thanks. I will follow your advice, as I believe you already know