A concept to Charge LifePo4 wihout BMS

[numberonebikeslover]

Dear all;

I want to share an idea of cheap way to make a lifepo4 pack, without having any BMS. I want to ask those who are good/experts in this, to find flaws and tell me is it a possible one or not. Kindly Discuss, because may be what I’m thinking can save a lot of money and effort.

Tell me that what will tend to balance readily during a charge without having a BMS: series connected Lifepo4 cells (Figure A), or Parallel connected (Figure B)?

Looking forward for answers because that will lead me to explain and share my concept.

Have a nice day

Naeem

 

[fechter]

Yes, this will work OK.
The only real problem is you have many connections. Connections can fail.

 

[dogman dan]

Wouldn't the paralell cells tend to balance, while the series ones not? I thought that was why the ping, and other lifepo4's were first paralell connected, then series connected.

 

[numberonebikeslover]

Dear All;
Many Regards!
Thank you very much all of you specially Mr. Fetcher for such a prompt response and who have a lot of contribution in this creative website.

What I conclude from the discussion is that parallel connected cells of LiFePo4 will tend to balance readily.

“My concept is that instead of having to connect cells in series to increase voltage, I would connect them in parallel to increase Amps. Getting the minimum 3.4 volts (approximately) I intend to make a voltage inverter/converter which I saw some 11 years ago. The circuit contained two power transistors (though I will use more) which converted dc to ac which is then put into a step up transformer which increased the voltage to the required range”.

The same kind is used these days in hour houses for electric power. It generates 220 volts out of a 12 V vehicle lead acid battery (100 to 200 amps) to power the house hold lighting and fan system during power failures (UPS, Un-interrupted power supply Unit).

Tell how much is the above strategy worth trying. Your helpful comments will be highly obliged.

It is worth mentioning here that I’m a man with a scarce monthly pay, so I can’t buy expensive BMS circuits, which are not available here in my country. Infact no one even owns any electric bike or car. So mine is the first one in our community which is currently run by Sealed Lead acid batteries (6v 10ah) x 9, giving me a range of 27-35 km, max speed 55 km, on flat 45 km.

Sincerely Yours
M. Naeem Zahid

 

[liveforphysics]

Naeem, you have the coolest bike on this entire forum, bar none.

If I were to ever go to Pakistan, it would be just to shake your hand.

Yes, that will definately effectively balance cells, and it's something that has been tried and true. Perhaps the very most reliable way to balance cells.

Like Fetcher mentioned, the downside is the amount of connections and wires.

However, for a setup with up to 12 cells in series, you could wire the cells up with a lead from each cell going to a Molex 20 or 24pin (depending on how many cells you are using), then make a pair of plugs to connect to that connector. One plug would be setup to join the cells in series, one plug would be setup to join the cells in parallel to balance. The downside would be that a molex connector is limited to about 10-15amps continous, but perhaps you could find another connector with the 2 times the number of pins as your number of cells that is rated for higher current loads. That would enable the balanceing to all happen just from just swapping a single plug.

You are awesome!!! Two huge thumbs up for you and your amazing bike, and your amazing skills and creativity!

Best Wishes,
-Luke

 

[numberonebikeslover]

Naeem, you have the coolest bike on this entire forum, bar none.

BEAUTI IS IN THE EYES OF THE BEHOLDER

I'm just a man, striving my way towards serving humanity. And i believe that all men are equal, it is just the efforts that they make towards achieving something, makes them different. I wish i could do better.

I can't find enough strength in my words to thank you enough.

Sir luke, about that switch you mentioned; i'm just going to connect cells in parallel and get 3.4 volt and then step up to 54 volt thats all. I guess there is no need for any switch or many connections except for connecting all the cells in parallel.

With warm regards

Sincerely
Naeem

 

[ZapPat]

Naeem,

I understand your goal of simplicity, but there is one problem with your voltage step-up idea: Efficiency.

A home made boost circuit with such a high step-up ratio will probably be pretty inefficient unless done very very well. This is mainly because you are going to have very big currents flowing on your low-V input side.

Ex: For a 35V output at 20A and a 3.5V input, you will need 200A at the input (plus some extra because of losses of course).

When working with such high currents you will need to study PWM circuit theory quite a bit, and come up with a very good design if you don't want your circuit to have a giant heatsink to cope with the losses. It can be done, but it would be challenge. A multi-phase design I would think...

If I were you I would go with the previously mentioned idea of using wire connectors to switch between series and parallel configurations. Much simpler.

WOW! That bike of yours IS technical art! You must be very handy and imaginative... not to mention you sound humble too!

Good luck!
Pat

 

[liveforphysics]

Zappat- He doesn't need to ever charge with them in P. Just connect them in P for an hour or so every few charges to balance the pack. You charge in series at normal voltage, you just use the P connection for the times you need to balance. The trick is just in finding a way to easily connect and disconnect cells.

For those of you who haven't seen this man's increadible creation, you have to see this thread. It will open your eyes to a new level of human resourcefulness, genus, and creativity.

http://www.endless-sphere.com/forums/viewtopic.php?f=6&t=3983

With much respect,
-Luke

 

[ZapPat]

Zappat- He doesn't need to ever charge with them in P. Just connect them in P for an hour or so every few charges to balance the pack. You charge in series at normal voltage, you just use the P connection for the times you need to balance. The trick is just in finding a way to easily connect and disconnect cells.

For those of you who haven't seen this man's increadible creation, you have to see this thread. It will open your eyes to a new level of human resourcefulness, genus, and creativity.

viewtopic.php?f=6&t=3983

With much respect,
-Luke

Good point Luke. I would still be quite carefull though, specially if he is going to use cheaper cells. Key points to watch out for:

- Never run the series pack down too low to avoid any weak cell from hitting LVC (meaning setting a conservative pack level LVC)

- Only charge the pack up completely if in parallel, since having no individual cell monitoring during series charging might permit one cell to be charged over it's max voltage if the packs gets too close to full charge. Bulk charging could be done in series, but I would limit total voltage somewhat bellow the usual 3.75V/cell (for LiFePO4), or have some way of monitoring Amp-hours in and out of the pack to make sure you don't charge in series when the cells are close to full charge.

Maybe I'm too prudent with these suggestions, but at the price we pay for cells it's important not to abuse them even once during their life. And cheaper cells do have more of a tendancy to fall out of balance faster, specially when pushed hard.

BTW, I noticed this simple and cheap Lipo High+Low voltage indicator which might be easily modified for LiFePO4 use by changing some resistors? I'll probably pick a few up for myself next time I order there to play with.