How much can 4x 100a BMS draw in series?

[Sethodine]

Pardon the newbie question, I tried searching the forum and couldn't find an answer.

I am currently looking to buy 48v 100ah of Lifepo4 batteries for a project, and I expect my maximum constant drain will be around 70 amps.

The 12v 100ah batteries all come with 100a BMS (with 200a surge), but the adverts suggest the constant drain shouldn't exceed 50a.

My question is, does this change if the draw is 48v from 4 batteries in series? Or do I need to build my own battery pack with a bigger BMS? Or is the 50% limit more of a suggestion than a rule?

 

[JoeHam]

The first question is can the BMS handle 48V ?

Some can, some can’t.

You will need to provide specific information (spec sheets) on the particular battery you are asking about before anyone can give a reliable answer.

The simple information is that in series voltage adds, current stays the same.

 

[rhino]

Much simpler if you get a 48V battery to begin with if your system voltage is going to be 48V and would likely be cheaper.

 

[Sethodine]

Much simpler if you get a 48V battery to begin with if your system voltage is going to be 48V and would likely be cheaper.

All the 48v 100ah Lifepo4 batteries I am finding are for golf carts, and they are double the cost of buying 4x 12v.

But Google has been getting basically useless lately, so maybe it's just not showing me other options. Are there 48v 20ah batteries I can run in parallel, or something like that?

 

[XO4001]

All the 48v 100ah Lifepo4 batteries I am finding are for golf carts, and they are double the cost of buying 4x 12v.

But Google has been getting basically useless lately, so maybe it's just not showing me other options. Are there 48v 20ah batteries I can run in parallel, or something like that?

Amazon.com : 48v 20ah lithium battery

 

[MisterSandals]

I am currently looking to buy 48v 100ah of Lifepo4 batteries for a project, and I expect my maximum constant drain will be around 70 amps.

100Ah / 70A = 1.43h runtime. What are you doing with these batteries?

70A x 51.2V nominal = 3584W wow!

 

[solarsimon]

1. You can't put 12V batteries in series unless the manufacturer specifically states you can
2. Drawing 70A continuous from a 100Ah battery is pushing things. Expect issues , unless it's very intermittently used
3. Go 48V from the ground up

 

[Sethodine]

100Ah / 70A = 1.43h runtime. What are you doing with these batteries?

It's for an electric tractor conversion, so it's got a drive motor + the power take-off for mower deck, snow blower etc. I probably wouldn't ever run both motors full power during normal operation, but it would be possible, so before dropping $$$$ on batteries I would want to make sure I am not gonna fry something.

1. You can't put 12V batteries in series unless the manufacturer specifically states you can
2. Drawing 70A continuous from a 100Ah battery is pushing things. Expect issues , unless it's very intermittently used
3. Go 48V from the ground up

A lot of the ones I am seeing are sold in sets of 4 specifically as 48v setups.

But after reading the responses here, it looks like I should just spring extra for the big 48v golf cart battery with the 200A BMS. It seems that should be more than adequate.

 

[MisterSandals]

It's for an electric tractor conversion, so it's got a drive motor + the power take-off for mower deck, snow blower etc.

3584W / (746W/ hp) = 4.8hp tractor?

 

[Sethodine]

3584W / (746W/ hp) = 4.8hp tractor?

It's a 1980s garden tractor. Originally 16hp kohler engine on a hydrostatic transmission. Mostly for mowing and snow blowing, but I also have a wood chipper, 48" tiller, and several ground-engaging tools.

The drive motor is 48v 1800w, mated to the original 30:1 gear reduction, which is more than enough power to move whatever I need. The power takeoff motor is 48v 2000w, and that powers just one implement at a time.

Electric motors are so much more efficient than ICE, so you can't really compare kW to HP straight across. GE actually made a 36v garden tractor starting in the 1960s, that even had a front end loader and forklift attachment options. Some are still running today.

 

[Cmiller]

My brother stuck a 20kW 48Vdc motor in a "garden tractor... (lawn mower with no deck). It goes 28mph, throws gravel like an insane madman, and burns up belts as well. (Yes, he still has the original rear end in it.)

He is currently using 100AH 48V LFP for his battery. His speed controller can draw up to 500A! But he is currently running without a BMS. We need to get his JK BMS with a balancer on the pack, as he has already ruined at least 2 cells from cell imbalances!

Morel of my story is this:

• Get a 48V battery! Get a 48V battery! Get a 48V battery!
• Did I mention.... Get a 48V battery!

Then for my other notes:

• You will need to be able to feed pretty high amperage to the motor to get the thing rolling! You will need a pretty high amperage BMS!
• I doubt you will actually find a 48V battery with a warranty and a BMS that will work out of the box for your project!
• If you DIY the battery, be sure to use a BMS, and an active balancer from the get-go!
• 70A will probably be borderline on enough power, even though my brother's setup is waaaaayyyyyyy overkill!
• For a reference on capacity needs, my brother can drive his "thing" about 22-25 miles on a full charge! (again, that is 100AH 48V)

For those who may be curious, here is a picture of his motor with half of the batteries in front and half below the steering columb. He has no hood on it yet, as the original didn't fit.

 

[Cmiller]

It's a 1980s garden tractor. Originally 16hp kohler engine on a hydrostatic transmission. Mostly for mowing and snow blowing, but I also have a wood chipper, 48" tiller, and several ground-engaging tools.

Hydrostatic transmission will probably turn pretty hard if you are using it still.... but you may not be.

The drive motor is 48v 1800w, mated to the original 30:1 gear reduction, which is more than enough power to move whatever I need.

Is the drive motor to be direct drive to the rear, eliminating the hydrostat? I am going to say that you will still sometimes need more than 1800 watts! just puttering along, you won't. But you are bound to sometime need some grunt to get out of mud, or pull a trailer, or even for when you pull that 48" tiller through the garden! That has to pull decently hard when you are in loose dirt and dragging a 48" tiller!

For a reference point on this, I converted a small 4wheeler to battery for my boys. I used a 2,000 EDIT*1,800 watt 48V motor off Amazon. I am currently running it off 2x Dewalt 60V FlexVolt batteries, so the motor is likely actually being overpowered vs. it's rating. I can't personally drive the 4wheeler (I'm 175lbs) because it doesn't have enough oomph to get me going, and I burned up the original 1800 watt 48V speed controller! (EDIT* now replaced with a 2,000 watt controller.) Gearing-wise, it goes ~18mph, so not geared way high. It won't even go through a hay field well with only a kid on it!

The power takeoff motor is 48v 2000w, and that powers just one implement at a time.

I doubt that will run your tiller!

Electric motors are so much more efficient than ICE, so you can't really compare kW to HP straight across. GE actually made a 36v garden tractor starting in the 1960s, that even had a front end loader and forklift attachment options. Some are still running today.

@MisterSandals reference 746W/hp above. I know you can't just directly reference that, but where the electric motors shine is instant torque! You still need X torque to run X amount of load. And your 2,000w + 1,800w =3800w total. That is only equivalent to about 5HP to replace your original 16HP ICE engine! That's only 1/3!

Not trying to rain on your parade, as I love playing with conversions! But just telling you what I have seen....

 

[Sethodine]

Is the drive motor to be direct drive to the rear, eliminating the hydrostat?

The original hydrostat was shredded. The entire tractor was junk, I only got it because it came with a front end loader attached to it that's going on my other tractor (22hp v-twin).

So I am deleting the hydro and direct-driving the 30:1 gearbox. Top speed at 3250RPMs was originally 7.2mph. The electric drive motor is rated for 3400 RPM so I should get similar performance.

With these tractors, the engine hp isn't the issue, it's getting enough traction on the wheels. When I am snowblowing I have my 180lb carcass plus another 300lbs of rear weights just to keep the wheels from spinning. The low-end electric torque will be a good replacement for the hydrostat in that regard.

I suspect 2000w may be insufficient for the tiller, but thankfully I can still run that on the gas tractor if that's the case. All the other implements shouldn't have any issues, as long as I gear it properly.

 

[Sethodine]

For comparison, I looked up the numbers and the GE Elek-trak tractor had a 1hp drive motor and their largest accessory motor was 2.5hp for the snow blower. This was considered comparable to a 16-18hp gas tractor. The 36" tiller used a 2hp motor.

 

[Cmiller]

For comparison, I looked up the numbers and the GE Elek-trak tractor had a 1hp drive motor and their largest accessory motor was 2.5hp for the snow blower. This was considered comparable to a 16-18hp gas tractor. The 36" tiller used a 2hp motor.

Interesting. Do keep us posted on how things turn out for you! As I'm interested in eventually doing a conversion on either a side by side of some sort (Polaris Ranger-ish or similar) or a tractor for using at home around our "mini-farm".

I imagine if your not after a speed higher than 7-10ish mph that will definitely help. But I tend to always want to be over-engineered rather than borderline. Lol.