wiring 2 LifePo4 36v 30 ah batteries to make 72 volts

[dirtbiketoebike]

Hello. Read a discussion forum but didn't really get the answer I needed. If I am charging my two 36v LifePo4 batteries seperate is it going to be a problem wiring them to 72 volts. I heard about the BMS can't handle it? Doesn't the BMS only work when your charging the batteries? Or because 72 volts will now enter a 36 volt BMS... burning it out? Thanks in advance....

 

[dnmun]

you need to be certain that the controller can handle the 90V of the two batteries in series.

 

[dirtbiketoebike]

I haven't gotten the wheel kit in yet but it is a 1500 watt motor rated to use 48-72volts . The controller is 35 amps. I want to use my two 36v 30ah x2=72v. Thks..

 

[dnmun]

do you understand that a 36V lifepo4 pack is actually charged up to almost 45V DC? do you know why we call them 36V?

 

[dirtbiketoebike]

No I didn't. Obsorbing...it in, newbie

 

[dnmun]

each lifepo4 cell is considered to be 3V nominal. this follows from the use of 12V lead acid batteries originally for ebikes, so that a 4S battery is 12V and a 12S pack is 36V but each cell charges to 3.65V full charge for lifepo4. this gives a fully charged voltage of 43.8V DC and chargers are sometimes set to 45V to fully charge and balance them under the BMS.

 

[wesnewell]

Lifepo4 cells have a nominal voltage of 3.2-3.25V, usually 3.2V. Considered fully charged at 3.6V per cell. A fully charged 12s 36V pack will be ~43.2V.
http://en.wikipedia.org/wiki/Lithium_iron_phosphate_battery

 

[dogman dan]

It's the controller rating you need to pay attention to, not the motor rating. Some controllers are able to handle a wide voltage range. Typically a "72v" controller can handle up to 90v of fully charged battery.

How the hell do you carry 72v 30 ah on a bicycle? Or is it for a bike?

Read a lot more, especially the wiki section first.

 

[Kent]

Hello. Read a discussion forum but didn't really get the answer I needed. If I am charging my two 36v LifePo4 batteries seperate is it going to be a problem wiring them to 72 volts. I heard about the BMS can't handle it? Doesn't the BMS only work when your charging the batteries? Or because 72 volts will now enter a 36 volt BMS... burning it out? Thanks in advance....

isolate each battery with a cut off switch and charge them independently. To speed things up, get two 36 volt chargers.

 

[granolaboy]

Assuming both your 36v packs have BMS's, you'll need to add some diodes to prevent current from back-flowing into the other battery.

From the Ping wiring guide:

How to connect multiple battery packs in series

Diodes need to be added in the circuit to protect the BMS.

- Voltage rating of each diode = maximum voltage of the battery pack
- Current rating of each diode = maximum current of the system

Diodes in this scenario don’t consume power when the system is working.
PingBattery supplies suitable diodes. Just contact pingping227@hotmail.com.

 

[dirtbiketoebike]

Thanks for all the knowledge. Gives me a much better understanding.
With the 72v 30ah battery question, it is a bike....When I said "I want to use my two 36v 30ah x2=72v." The batteries should be here in the mail any time now. Got the batteries at good deal. I have to admitt the bikes probably going to be too heavy and unstable. LOL, I tied on two 25lb weights to the bike today.... (about the same weight as the lifePo4's description ) I have to figure maybe another 15-20 LBS for the kit? I Didn't like the feel of the weighted bike but can be managable for on road use. I know all sounds little crazydumb , but my employee is using lead batteries for his on road bike. And before I ordered to experiment some, he said he buy the batteries if I don't want them. I'll wait to get them in but think 15-20 ah might be more suitable for offraod. Much appreciated for the info!

 

[dogman dan]

Well, you sure will have some range anyway. But if you cruise at full throttle for more than ten miles continuous, it can melt the moderately sized motors.

One 36v battery should be tolerable to carry, and you won't melt the motor that way. I'd suggest a trailer, but assuming 40 mph that won't be so good either.

Now that you are here, we can help you with the next try, once we know what your goals are, and what the budget is. Depending on what kind of cells you have bought, it might even be possible to build yourself something practical from what you have. 48v 30 ah with 24v carried in each pannier might be ok. Traveling 30 mph on 48v won't melt your motor. You'd have 30 miles of range at 30 mph, or more.

Then you still would have enough cells to construct a 24v 30 ah from the remainder.

For offroad, many of us use the RC lipo batteries. Dangerous, but very powerful in a small and light size. 72v 10 ah does not wreck the handling of the bike if you can carry it in the front.

 

[Ghost (bike)rider]

I have carried 2 x 48V 20Ah batteries (one in each pannier) and put the voltage output in series (to get 96V) but kept the charger connections in parallel (so charging both batteries with a 48V charger). Yes, its heavy on the bike but it goes like a rocket off the lights. If you are doing something similar, here are a couple of tips:

1. Get tyres that have strong sidewalls and handle high pressure. I use Specialized flakjacket.
2. Keep the rear tyre at around 80PSI - that way it is stable (not too much sideways wobble when cornering) but not too sensitive to hard hits.
3. Check the spokes regularly and keep the wheel true. 70kph+ is enough to loosen them with any imbalance

I have to say, I dropped back to 48V because the torque was breaking spokes - even 12G. I think 72V would be a nice compromise.

 

[dnmun]

except it is not 96V. it is almost 117V DC. very few mosfets can handle 117V and almost none of the controllers have caps rated higher than 100V. so it is either bogus or not really charged or you would mention what controller you had to find to do it.

it is not fair to someone who does not know anything like this guy to imply that it is possible to just double the voltage on the controller. bad enuff he already got all this other disinformation.

 

[dogman dan]

Yeah, 96v nominal, It would be hard on a 72v controller since that would charge to 116v or so. 100v caps, so he'd be pushing it hard most of the ride.

But the OP can run a 72v controller just fine on his two 36v batteries. What I can't imagine is going 40 mph with that much weight on a bike frame. Unless it's a very strong cargo bike, preferably a longtail. 40 on my longtail with 50 pounds of cargo should be doable, but I haven't actually gone faster than 35 on it. Hard on tires though, so you'd need good ones for sure.

And then he's talking about trail riding. OMG.

 

[dirtbiketoebike]

Now I have alot to think about. I been riding dirtbikes most of my life but am without one now.. I accidentally ran into these ebikes a short time ago. Looks fun. Closest thing and more affordable than dirtbike. LOL,I have a ton of utube hours, but still alot to learn. Glad to see brushless motors and battery technology advance. I am seeing the R/C hobbies too converting back to battery and ridding of the nitro fuel.

Even riding dirtbikes 30 mph in the right condition can get the blood flowing but the kit I'm getting said it can go 72kmh. Not sure if that's pedal assisting the motor and going down hill.I woud want the bike to be able to pedal back if the battery died. "not a tank" but good range of maybe 20-30 miles...so the 2 36v 30ah batteries not going to work,too heavy.Likely will give that other 36v to worker and keep one. My riding would be all around. I will pedal to get excercise but want it to be able to move me fast if Im too lazy or tired.
So is it a sales pitch that the manufactor is advertising 48-72V kit? Curious because Dogman,you mention that the motor might melt with the moderately sized motors? Only reason I'm leaning toward 72v because it seems like all the fast ones on utube have high voltage batteries and higher motors....but I don't know the full reasoning and maybe the 48v will do fine. Does Higher voltage equals higher rpms and bigger wattage motor hub produce more torque and RPM's? Well I'm stuck with the 1500 watt kit coming, but the batteries I have options still.Taking them apart and making different series...I can do it with some more advice. Just not sure if I have that kind of time. The ones I saw on utube from laptop batteries have too much time in making the batteries. But I guess this 36v 30ah battery has most of the work like tabs are already welded if I can make the pannier close to what they have soldered up? If I took apart the 36v and made it into different voltage ,would I have to buy the proper BMS to match ...like 24v ,would have to buy a new 24v BMS too? Or a charger like the !MAX B^ digital charger in R/C Hobbies type charger?
Ok too many questions....I going to slow down and ask in different posts. How much are you guys charging me per hour? LOL