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Battery pack explosion question...
#1
I'm worried that I will explode my 4S4P battery pack in the back of my motorcycle!!!!   Here's the details-

So I've built a 4S4P pack to power my CPAP machine while on a motorcycle camping trip.  (I will be riding about 6-8 hrs a day)

Got it all Soldered and it's giving me 15.5V, which is good. 
So I got it all connected to a cheap BMS I found on Amazon, and the output from that board is only 9.3V !!!!

So I completely disconnected the BMS board.   I'm thinking of just connecting it directly to the bikes charging system via step up converter (dialing in 16.6V).    This would be a steady voltage of 16.6V, and I could adjust the current too, (this board has 2 potentiometers-  1 for Voltage, and 1 for current)  Not sure how much current would be optimum..

I was thinking of adding a small voltage meter and checking this periodically while I ride, so as not to over charge the battery-   but I'm deathly afraid of a giant bomb going off in my rear case where the battery is housed..

Any help, or advise with this would be greatly appreciated. 

RD
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#2
You can't directly charge lithium like you do with lead acid. You can't set the voltage and forget it, type of deal. It has to charge through the curve. It has to charge through Constant Current first. Constant Voltage isn't needed at the end of charge, but is recommended to extend the life of the cells and get more steady capacity out of them. So connecting directly to your charging system from the bike isn't a good idea.
But, you can use a solar pwm/mppt charge controller. It will just be set to a very specific input voltage, unlike what solar can do. Probably in this case, pwm would be preferred over mppt as mppt will try to change the output voltage of the bike charger. PWM won't do this.
But both PWM/MPPT will charge according to the curve, CC/CV modes.

Overall you won't have a "bomb", per se, as long as the charging voltage stops. If the packs max voltage is 15.5V and the charging voltage is 15.5V, it won't blow up. It could if the voltage went up to about 18V (4.5V / cell) or higher.

I would recommend the PWM solar control charger myself. I'm sure others will chime in with their views shortly.
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#3
You can charge it like you said yes. That board have CC and CV function and setting it to lets say 4.1V*4 will work fine!
= Any normal Boost converter with proper CC CV function works fine. They are like any solar charge controller without the MPPT function.
BUT
You need to have a BMS in between to take care of any eventual balancing and protection like you said.

Korishan: PWM controllers takes down the voltage and will not fit here unless you have one with a boost function.. i have not seen any?

Why your BMS did show that low voltage on output I dont know but can be due to its shutting off?
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#4
It will work as long as the cells stay in balance. That will probably be the case for some time with new cells but you have to monitor the situation when not using a reliable BMS. That said using no BMS or a cheap and nasty BMS is probably the same as you have to monitor the work done by the cheap and nasty BMS anyway.

So either put your hands deeper into your pockets and buy a decent BMS (I can't recommend one for your application, not sure what would be a decent choice) or make sure to monitor the cells as they get older. Or just take a balance charger with you, they will work from the onboard supply as well and can balance charge the battery. If sized properly the battery won't run down all the way over night so you won't have any need for an automated low voltage cutoff for battery protection by a BMS. In this case you can go without a BMS completely.
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#5
Thanks for your feedback Daromer and Korishan!

These (below) are the two Boost converters I purchased from ebay. I think I'll use the smaller 5A one. I was planning on setting the voltage to 15.5, but not sure what current to set it to. Any help on this would be gratefully recieved!!

Since I don't have a Li-ion charger that takes a 12V input (from the bike) , and I'm leaving in two days from now, this constant voltage and current is probably my only option..
Since I'm not worried about the long term life of my batteries, I don't think it will be a big deal not to balance charge them. Only need them a few weeks.

https://www.ebay.com/itm/Boost-DC-DC-Con...2749.l2649

https://www.ebay.com/itm/DC-DC-CC-CV-Buc...2749.l2649
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#6
The second one is a buck converter, you can't use that one. Only the first one is viable. Current should be the usual charging rate of the cells you are using times four.
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#7
Moste cheaper BMS systems from China would work just fine. Just make sure it can cope with the current. I have built many many small packs with such devices and they still run today several years later Big Grin And yes they are charged and abused with normal DC supplies and cheap BMS.

I would never use such things for powerwalls or big installs and or ebikes with high current demands. In your case its about your life so you should consider and atleast get and more importantly test it!

Lastly always have a backup solution with you if this one fails! A solution that is tested!
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#8
(06-12-2018, 02:04 PM)daromer Wrote: Korishan: PWM controllers takes down the voltage and will not fit here unless you have one with a boost function.. i have not seen any?

I know. ...... Ahh, reread his first post. For some reason I was thinking his motorcycle was outputting 16V and he was charging his packs to 15.xV So yeah, the PWM wouldn't work in this case. I gotta stop replying to messages when my eyes are still bleary from being sleepy Tongue
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#9
I showed the wrong link to ebay. i do have the first one, but was hoping to use my smaller one.. https://www.ebay.com/itm/DC-DC-Boost-Ste...2749.l2649

It only allows a voltage adjustment, and not current.

So DarkRaven, what your saying is 2600 mah battery X 4 in series = 10,400 mah or 10.4 amp hrs for charging.??? seems really high.

The smaller board only goes to 5 amps, but not sure it'd be good to go over about 2 amps. thoughts??

To add to my previous comment, the battery is a 4S4P pack, so the approx current to charge this is what I'm looking for..

Thx

RD
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#10
You can't use this one because of the current limiting feature. Or rather the lack of. You can use it as a power supply within its load limit but not as a battery charger. If you connect a battery across its output then the current will go far beyond the maximum 5A. In other words you will just overload the converter and the output voltage will drop massively or it will turn off if it has working safety features implemented. It will be close to a short circuit on the output of the module.

Your calculation is correct though, more or less, with slight misuse of the units Big Grin
If your cells are 2600mAh and we assume 1C as a reasonable charge rate, which is the case if these are quality cells, then it will be 2.6A x4 = 10.4A because there are four cells in parallel. This is not high at all. It's reasonable for new cells without having the datasheet at hand. In the datasheet the manufacturer will have specified what they think is a reasonable current. Of course you can opt for 0.5C or 5A or even less with the charging time being proportionally longer.

Talking about really high currents, "fast charging", 2C and more, we have to go to 20A at least so we are still far away from really high Wink
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