Battery Pack Question

[moderator007]

I am building a hunting light for a friend of mine. Modifying his old nite lite that used lead acid battery. I am going to use dx 3.7v batterys to make the pack. Wired 2s5p 7.4v for a total of 10 batterys. This should give him about 6hrs runtime at 2amps of draw at full power. I am using a pwm circuit with pot to control the brightness and longer run times. As he doesnt need full brightness when walking through the woods.

Now my question is if I use protected batterys with pcb on them. Would this not prevent a short circuit of 1 battery in the pack. Seems like to me if 1 battery goes bad there is less of a danger of explosion:poof:. Because as the battery goes bad the pcb will shut that battery off and all the rest will still work fine. And also each battery would drain until pcb cuts them off one by one in the 2 parallel sides. Until there was no battery on one of the parallel sides still holding above its pcb voltage. Would this not be safer. Charging the batterys with the pcb should protect the batterys from over charging in the same way, individually per a side. Is there some reason that i dont no about that would make this a bad idea? Comments are welcome.

 

[KiwiMark]

Now my question is if I use protected batterys with pcb on them. Would this not prevent a short circuit of 1 battery in the pack. Seems like to me if 1 battery goes bad there is less of a danger of explosion:poof:. Because as the battery goes bad the pcb will shut that battery off and all the rest will still work fine. And also each battery would drain until pcb cuts them off one by one in the 2 parallel sides. Until there was no battery on one of the parallel sides still holding above its pcb voltage. Would this not be safer. Charging the batterys with the pcb should protect the batterys from over charging in the same way, individually per a side. Is there some reason that i dont no about that would make this a bad idea? Comments are welcome.

The PCB will give you a measure of protection against bad things happening, but I don't like the sound of relying on it to stop over-charging or over-discharging (nothing is 100% reliable).
I don't know that it should be an issue though - all 5 cells wired in parallel should have exactly the same voltage while charging & while discharging (and while sitting idle for that matter), so none should be over or under voltage unless they all are. You should be able to charge them as if they were 2 cells with each having 5x the capacity of what 1 separate cell would have.

 

[Potato42]

In a similar pack design for radio controlled vehicles, there would be a separate set of wires for "balancing" the pack. The individual cells will not always charge and discharge at the same rate throughout the life of the pack. Even if they are balanced perfectly beforehand. For this reason, the balance tap allows individual cells to connect to a charger so that voltage can be kept as close as possible between the different cells. Of course this requires a charger capable of "balancing". Do some searches on lipo balancing to get some more info on the subject. Personally I would never put a pack together with lithium batteries without the ability to balance cells. For quick refrence, a decent charger that can balance cells like I described might run around $40-50. If that seems like too much of an investment, consider using a different battery technology like NiMh.

 

[moderator007]

kiwimark, do you thank that wiring a additional pcb for 7.4v for the hole pack would make it even safer. By the way I am planning on wiring in a balance plug to the pack to charge each 3.7v side balanced. Just dont want this pack to go :poof: on my friend. He carries it on his side with a belt. If something happened it would not be good for me or him. Is there some safer way to do this without spending alot of extra cash?

 

[Potato42]

As long as the cells are not overcharged, over-discharged, and remain balanced, everything should be just fine. If you plan to do the balance plug then I wouldn't worry too much.

 

[moderator007]

potato42, did not see your post until i hit the submit button on my reply to kiwimark. I will be purchasing and wiring up hobby charger for him to charge the pack. Using some sort of external plug. As he liked not having to take the batterys out to charge them. Just plug and play. Done lots of research on batterys and packs. I have a turnigy Accucel-8. I just dont know of a better way to get a 7.4v 12000mah pack for about 50.00 bucks. And very light wieght compared to lead acid or nimh.

 

[Potato42]

Yeah they get pricey really fast. I used to race 1/10 scale onroad 12 or so years ago. Back then 2000mah PACKS were hot, and could run $50 each. Now we can buy a 26650 that's twice the capacity for a quarter the price and a fraction of the weight. I guess we're doing better than ever!

 

[moderator007]

What about adding a additional 7.4v pcb for the hole pack. Doesnt need it or does it add any safety. I dont hardly trust the dx pcb. Even though its always worked on my other batterys.

 

[alpg88]

do not use protected calls to make a pack, use unprotected and ballance circuit, don't use cheap dx cells, use good japan\korea cells with tabs, solder 2p5s, and hook it to pcb designed for 5s cells, wheather you got 2p or 3p is irelavant, as long as pcb supports capacity, don't solder li ion cells, only tabs.
if one cell overdischarges circuit will be broken, and your pack will shut off.
cells, pcb

for perfect balance use pcb with Equilibrium Function, it will equalize state of chage in all cells in series, by discharging cells that have higher state of charge, doing that pcb will turn curent into heat, lots of it, (can't have it sealed), but keep in mind, 99,9% of li ion packs don't have that function, (laptop, cameas,....) yet work just fine, most of the time.

 

[moderator007]

Alpg88, you said if one cell overdischarges circuit will be broken. What about the other 4 batterys on that side that are still working they are 5 parallel on 2 sides. A dead shorted battery would be what a thought would be my only concern. But the pcb on each and every battery should prevent over discharging any one battery. Also i know that soldering to the battery is a no,no. But i have seen this done with no ill efects. Even though i would not tell anyone to do this. CAUTION Do not try this unless you know what you are doing. You have to know how to use a soldering iron and how to prep the battery.

 

[alpg88]

if a protected circuit in a cell shuts off for whatever reason, cell that is paralelled will unfreeze that pcb, (you'll basicly have either overdischarged cell despite pcb, or damaged pcb that might no longer work). also cheap dx protected cells have tendency, (as it was proven here many times) to fail before their time, there was at least 1 case here on cpf that ultrafire protected cell blew up, ripping apart aluminium light. also having 10 cheap inferior cells with "minds of their own" in not a good thing. also if 1 or 2 cells in paralell shuts of, and lets assume pcb of working cell wont unfreeze it, you have overloaded cell, (unless the total draw is less than 2c of single cell.
i myself soldered li ions before, ill effect is not visible right away, but very soon cells fail, i lost 2 out of 4 packs i build that way. that is why based on personal experience i don't recomend this. especially soldering protected cells, you have a chance of damaging pcb, wheather it would just storp working, or shorts out and gives you fireworks, depends on how lucky you are.

p.s. you really don't want to spend all that effort and money on making a pack, to see it loose few cells, and loose most of the capacity, belive me, i've been there, at the end you'll still redo it right, but for 2x the cost.

if you insist on using dx cells with pcb, at least build a holder so you can easy swap damaged cells,

 

[moderator007]

Alpg88, I appreciate your replys. I did see this thread (when he posted it) of the flashlight you are refering too. Very bad. That is why im trying to make the (safest affordible pack) i can. Maybe those three words dont belong together.

if a protected circuit in a cell shuts off for whatever reason, cell that is paralelled will unfreeze that pcb, (you'll basicly have either overdischarged cell despite pcb, or damaged pcb that might no longer work).

I want you to correct me if I'am wrong here. But doesn't the pcb see the voltage and current coming from the battery tabs on the actual battery. Not the tabs on the outside of the battery that we see. I thought that was how it made the break in the connection when you over discharge. Its like a ic controlled switch between the outside and inside tabs.

If it does turn the pcb back on, wouldn't it be in a sense charging it from a higher voltage cell in parellel.

 

[alpg88]

I want you to correct me if I'am wrong here. But doesn't the pcb see the voltage and current coming from the battery tabs on the actual battery. Not the tabs on the outside of the battery that we see. I thought that was how it made the break in the connection when you over discharge. Its like a ic controlled switch between the outside and inside tabs.

If it does turn the pcb back on, wouldn't it be in a sense charging it from a higher voltage cell in parellel.

not really, try this at home, take any protected cell, discharge it to shut off point, or simply short it, than mesure V, you'll get 0, now connect any working cell parallel to first cell, now mesure the voltage on first cell.

as for charging it from a higher voltage cell in parellel, not really, thou it looks that way. chargers are 4,2v (most of the time) no way cell next to discharged one will be 4,2v, it pbly be a bit more v than first cell, they are discharging in parallel .

 

[alpg88]

I want you to correct me if I'am wrong here. But doesn't the pcb see the voltage and current coming from the battery tabs on the actual battery. Not the tabs on the outside of the battery that we see. I thought that was how it made the break in the connection when you over discharge. Its like a ic controlled switch between the outside and inside tabs.

If it does turn the pcb back on, wouldn't it be in a sense charging it from a higher voltage cell in parellel.

not really, try this at home, take any protected cell, discharge it to shut off point, or simply short it, than mesure V, you'll get 0, now connect any working cell parallel to first cell, now mesure the voltage on first cell.

as for charging it from a higher voltage cell in parellel, not really, thou it looks that way. chargers are 4,2v (most of the time) no way cell next to discharged one will be 4,2v, it pbly be a bit more v than first cell, they are discharging in parallel .

 

[moderator007]

Alpg88, would this not be almost the same as if i did not have a pcb on each and every cell. And only one 7.4v pcb connected to all batterys. I was planning on using a 7.4v pcb on the hole pack also. Didn't know if you had read that above. Seems it would be more safer to me but im just not sure. Have thought about possibilities that could happen. But im still just unsure. Do you think it would be better to remove the pcbs on all the batterys and hook them up normal with the one and only 7.4v pcb to the pack.

 

[alpg88]

. Do you think it would be better to remove the pcbs on all the batterys and hook them up normal with the one and only 7.4v pcb to the pack.

i think it would be better not to buy dx protected cells, and buy good quality unprotected ones, with tabs.
7,4v pcb will only work for 2sxp set up, but since you said 5s2p than no it wont work, you need 5s pcb,

 

[KiwiMark]

i think it would be better not to buy dx protected cells, and buy good quality unprotected ones, with tabs.
7,4v pcb will only work for 2sxp set up, but since you said 5s2p than no it wont work, you need 5s pcb,

I believe Moderator007 IS talking about 2s5p - so it should work. And I agree - 10x good unprotected cells with tabs would be the best approach here. The good quality cells are more likely to stay in sync with each other and keep performing well for longer, with a protection PCB to cut off the battery when the voltage falls too low this should be a safe & reliable battery (but less so if using cheap cells).

 

[alpg88]

damn, you right, my mistake, idk why i thought we talking about 5s2p, in that case yes 7,4v pcb will work fine.

 

[Footleg]

not really, try this at home, take any protected cell, discharge it to shut off point, or simply short it, than mesure V, you'll get 0, now connect any working cell parallel to first cell, now mesure the voltage on first cell.

as for charging it from a higher voltage cell in parellel, not really, thou it looks that way. chargers are 4,2v (most of the time) no way cell next to discharged one will be 4,2v, it pbly be a bit more v than first cell, they are discharging in parallel .

Surely with parallel protected cells, as the voltage across both cells will be the same, then when the first cell reaches the point that the pcb disconnects it due to low voltage to other cell(s) in parallel will be so close to the pcb cut-off voltage that they will not have enough charge to turn on the pcb in the other cell? So the cells will turn off individually as they reach their cut-off voltage and very quickly all of them will have shut down. The pcbs will reset when the pack is next charged?