battery, bms and controller solution

[promethious]

hey all, perhaps a silly question (and please forgive my lack of pronunciation as it isnt my strong suit) im sure there has been an answer already, but it would be easier for me due to time if i could just find someone to answer this question... i am going to build my own lithium battery, and i found a half decent supplier (seemingly so far) of the cells which are rated at 3.7V 30AH... the first pack will be a 12V system for running lights and other electricals, however 3.7X4=14.8V ...higher than the 12 volts i need.. so with a BMS built in, what controller would i use to regulate the output so i could use it with my 12V system?
any ideas or suggestions would be greatly appreciated, thanks...

 

[dnmun]

that is not what a BMS does. you should study more before building a battery pack.

you just have to use the 14V output. should not make any difference to the light that you said requires 12V.

 

[promethious]

yes i realize i do need to study/learn more.. kind of part of the reason for coming to a forum like this ...but thanks, my question i guess wasnt expressed properly.. so how DO you limit the output level easily on a battery for 12V systems?

 

[dogman dan]

12v is generally designed to run at highest voltage you charge lead batteries to, which is 13v or higher.

You don't need to sweat it, your 12v stuff will be fine on 13 or 14v. As soon as you start using it, you will see a drop to voltage in the 13's as well. But if you are planning to use cells that have a nominal voltage of 3.7v, then they charge to 4.1v at least. That means 3 cell is a 12v pack, not 4. Lifepo4, which fully charges to 3.6v takes a 4 cell pack, but the other lithium chemistries use 3s for 12v. It may be a bit weak though, if your 12v stuff cannot run on less than 12v. You would want to stop at about 11v, so if it will run on 11v you are good.

If you needed to get from 24v or whatever to 12v, then you would use a dc-dc voltage converter.

No dumb questions here, just uninformed ones. Till you learn more of the words, it's dang hard to even search for a thread with the answer. Now you can search for dc converter threads to learn more about them.

 

[promethious]

thanks dogman, and yes, the lingo is... something i need to learn lol... well im trying to decide between these 3.7V i found and some 3.2v cells.. im just... wanting to make sure i dont damage anything overall with the 12v system... the lights, GPS, MP3.. and an additional plug in are all on 12V.. so thats my end goal.. right now theres a 12v 7ah SLA battery there.. so i wanna cut down the weight, and increase the capacity..

 

[wesnewell]

A 4s rc lipo pack will charge to 16.8V. I use one to start my gas generator without a problem , but I'd be leery of using one for a 12V system without check the specs of the devices of the system to make sure they can take that much voltage. Your best option would be a 12v lifepo4 4s pack. It is designed for 12V systems and will have the proper voltage. You can find them on ebay are you can make your own.

 

[promethious]

thanks wesnewell yea i considered using lipos.. was using them in my airsoft stuff.. not nearly as bad as people think they are so long as you use them as directed and take proper safety of them.. but for this application im thinking i wanna stay/go with Lifepo4 instead.. a little more forgiving in the application.. although the basics are still the same.. atleast in application they are..

 

[dnmun]

do you intend to use this battery as the traction battery or it just for these lights? you can use a DC-DC converter from a 36V lifepo4 pack to make the lights run and use the 36V pack as the traction battery.

then you can leave the DC converter turned off if you don't need the lights or the 12V.

 

[dogman dan]

4s then, 4 cells in series will work good with lifepo4. FWIW, it's kind of easier to match 12v with lifepo4. 3s lico or limn is slightly low with 3s, and way high with 4s.

I take a little 4s 10 ah lifepo4 pack with me on camp trips to run lights in the tent, keep the Ipod music going, charge a phone. Works fine with anything that uses a cigarette lighter plug.

 

[promethious]

Dogman

yea, i was kind of hoping that a 4 cell pack might do the job, and then be able to use it for camping as well just by using the lighter plug-in's or something similar..
im assuming you have a escooter as well? just an offbeat question here...i was thinking about adding a windscreen/windsheild to mine... just not sure how much that will affect the resistance or if its even worth doing without causing havoc on the battery capacity overall...any thougths? ideas?
this is a pick of the bike i have (well same style anyhow, almost identical) still have to get my pics off my camera, but i need a new cable for it first

 

[dnmun]

if you expect to charge your 4S lifepo4 pack by plugging it into the lighter you are in for a big surprise. is that scooter 48V SLA or do you have the batteries in the saddle bags?

 

[promethious]

Dnmun

lol i guess im having trouble expressing my thoughts again.. no, im not going to CHARGE the pack with a 12V lighter.. i intend to have a !@v lighter plug for plugging in an item such as my phone or ipod..

and the bike is running 72V... currently yea, theyre SLA but have my lithium cells on the way... (and got an additional 6 cells for free as samples heh) ...so ill be swaping the sla's out for the lithium soon enough
and no the batteries arent in the saddlebags.. theyre in the under the seat

and sorry, i missed your earlier post i think, just saw it now... the bike has 2 batteries.. the traction battery is the 72V... there is another battery, 12V for the lights and stereo and the 12V plug in adapter for things like phone, and mp3 player or...well anything really.. so ill be swapping out both (as they are SLA for the lifepo4's once they come.. which is kind of cool, and has me excited as the 12V swap is less than half the weight pretty much, but has 3 times the AH... and the 72V is again uber light and has double the AH... with the current set up im getting on average a little over 60km range so far... (done twice so far in windy and cold air so im guessing it might be a little better actually on a normal warm day) but with the new batteries i cant wait to see the results...

 

[dnmun]

yep, i tried to drive my little ZENN car today on it's aging SLA pack and i had to give up and turn around. i cannot wait to finish building this ping pack to get it in the car.

but you may not have seen the threads on using an AC adapter, the wall warts for charging laptops and other devices.

you can use those off the high voltage pack to get to your 12V and you can even dial in the voltage you wanna set it to by adjusting the setpoint of the output voltage and then use if to charge your battery but you would need some current limiting feedback.

that is what i meant about plugging the battery into the lighter plug since the voltage difference between the 12 auto and the lower battery voltage when discharged would cause huge currents to flow through the lighter and it would just blow the fuse instantly. but it could be done if you used a pwm type current control and used that to limit the current going to the battery. i see them on ebay for like $12-15. good up to 80V DC.

 

[dogman dan]

Maybe I misled a bit there in my post. I was saying I discharge my 12v lifepo4 with the lighter plug. I charge the pack with an RC toy 50 watt charger after unplugging the lighter socket.

The lighter plug works fine with my car ipod charger, car phone charger, etc. The pack won't run big stuff for long, but it works great for a small lightbulb, or charging the ipod while it plays through small speakers in the tent. I haven't tried mine with a small AC inverter, my cells are pretty trashed. But a new pack should be able to charge a laptop, or even heat coffee.

I have a longtail cargo bike I use to go camping, so I can't answer your question about the windscreen. But if it does slow you down some, it may still be worth it for more comfort, especially when it's really cold.

 

[promethious]

well i think i am a happy man, at least for the moment lol... have been in contact with several (way too many) battery manufacturers in china... anyhow, have some samples coming so im that much closer to switching to lithium packs
now i gotta figure out how im gonna do this.. im going to try the 12V pack from prismatic cells.. this way i can make it removable easy enough and bring indoors when not in use... but im on the fence for the 72V... cylindrical or prismatic... pros and cons? anyone have any thoughts/comments?

 

[ten.ring]

12v electronic devices operate between 10.5v and 15.5v, most will be ok up to 16v. conventional lamps are pretty dim at 10.5v. most l.e.d. lamps will work with 10.5 to 30v.
your car battery with engine running, battery at full charge and no dc load will be 13.8 to 14.8 any load will bring it down into the 13.?? range. lead acid batteries are equalized at 15.5 to 16.0v

if it were me i'd use the accessory battery space for adding amp hrs to the main battery. and I would use a dc to dc converter (rated for at least 50% more than max accessory load). this would extend overall range when lights are not needed.

give us some detail about your battery samples, who did you get them from, what batteries are getting and how much did you pay including shipping.
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