Parallel Two 72v 20ah Battery Packs Increase Amps Available

I am getting mixed answers about combining two battery packs. I have two 72v 20ah with a 40a continuous 120a instant BMS. They are
3.7v 26650 5000ma LiNiMnCoO2 cells configured 20s 4p. I want to power a 24 fet 80a controller. I want to connect these two packs in
parallel to give me more amps and capacity. Is this correct ? I will charge the batteries separate. I am using Anderson plugs for
quick connect / disconnect. I am making a Y harness to connect to the controller. Anderson 50a plugs from the batteries to a 80a
plug to the controller.
Please let me know because I am getting mixed answers. Thank you

I've never done this so feel free to ignore anything I might suggest. When the two battery packs are connected in parallel, they must be EXACTLY the same voltage, or the high one will try to charge the low one, and it will do it very rapidly.

I don't recommend this for anyone at any time, that being said...when the moment of truth arrives to connect the two, put a jumper wire between both negative poles, and then connect the positives together through a simple resistive load, like a 100W filament light bulb. That will slow down any equalization current. Only after they have sat connected like this for a while should you remove the bulb and connect them to a pairing harness.

That is what I would try, but...even then, it could get the wires very hot, and it may even damage some part of your expensive battery. If anything looks or feels hinkey for any reason, yank them apart. Of course do this outside of a house on a concrete driveway, and wear gloves plus eye protection.

Yes, I have scars.

No problem. Just make sure you do charge separate, and you never connect them unless the voltage is similar, within about one volt difference. Ideally have them exactly the same, but in my experience, one volt difference does not result in that big a flow if you do not have a bms. no wires will get hot.

If you have a bms though, you DO want to get them closer, so the flow does not affect the bms. Easy to do in the field. Check voltage, plug in the higher voltage one and ride till you see the voltage you need on the high pack, then connect the two.

Do make sure your plugs have the polarity right, I also have the scars from messing that up. Andersons make it easy to make a Y harness and mix em up. Just be sure you put the black plug on the black wire. Always, with any connector, confirm polarities before with a voltmeter before the first use.

Yes, as long as the bms's play well together, it should be fine. I've been running three 72v paralleled up for quite a while with no problems... But check things a lot. I had one time where one pack wasn't connected like I thought it was, leaving it fully charged while the other two got used. I noticed it during charging, but if it had gotten plugged back together like that it wouldn't have been good with the voltage difference (10ish v). Also if you use fuses for each pack, check them a lot, as a blown fuse leave the other pack working hard, and different on voltage for the next hook up.

voltron and dogman dan

We are almost there. i want you guys to understand that both of these packs are identical and both have a 40a continuous bms.
I want to hook them together paralley to power my bigger controller. 80 amps. Are the amps cumulative with the bms or is the bms
going to hold back the current. With no BMS it is definitely cumulative. 40a + 40a = 80a. But i want to make sure with the bms in each
battery it is still 1 + 1 = 2. Not 1+ 1= 1 with the BMS. I don't want to cut the packs apart and remove the BMS in each. i am making a Y harness
to make it easy. battery A has 50a anderson plug same as battery B and that will connect into my 80a plug to the controller. I will make sure the
volts are close before connecting and always charge seperately. To spinnigmagnets: It is parallel not series I got my 72 volts I need more amps.

Thank you

One more reply and I think we got it guys. If all good this is going to be bad ass. I will send pic.

I have successfully hooked up LiFePo4 to both LiPo and LTO.

Reason for doing this is that LiFePo4 has a very good price per wh, where as the other two have a very good price for C rating. This way, I can get very high burst power, at a very low price.

Be aware that it's not just connection time that current flows between two battery packs. If your packs have massively different internal resistance, during periods of pull down, the pack with lower resistance will provide most of the power. When it "bounces back", the pack with the higher resistance will want to bounce back to a higher voltage, and will then feed as much power as it can to the lower IR pack.

You would be surprised at how many amps can travel over a 1v difference. On 20mOhm pack, a 1v difference will have a 50amp current. I would advocate an even smaller voltage difference before connecting up packs.

tjgaryiv60 said:

Are the amps cumulative with the bms or is the bms
going to hold back the current. With no BMS it is definitely cumulative. 40a + 40a = 80a. But i want to make sure with the bms in each
battery it is still 1 + 1 = 2. Not 1+ 1= 1 with the BMS.

Click to expand...

Sorry, missed this part.

If (and this is a big if), the internal resistance of both packs are identical, then yes, 1 + 1 = 2. You will get pretty close to double.

However, if the internal resistance is not equal, then in an absolute worst case scenario, 1 + 1 = 1.0000001.

The pack with the lower resistance will always provide more power. It doesn't quite work this way, but for simplicity's sake, let's say one battery provides double the power of the other, no matter what you want.
So if you want:

30A, battery A will give 20, battery B will give 10.
60A, battery A will give 40, battery B will give 20
61A, BMS A will trip, transferring all load to battery B, causing that one to trip as well, meaning you have no power.

Does that make sense?

As long as the connection does not break, each pack will pull approximately half the current. Each bms will see half the amps. But do be aware, if you romp on it enough, one bms can trip, leaving the other with all of it. This will apply only if both packs are at the same state of discharge, and are both healthy. When they start to wear out later, the still strong pack will pull more than the weak one.

YOU NEED TO STILL BE SOMEWHAT CONSERVATIVE WITH YOUR DISCHARGE RATE. So if each bms trips at 40 amps,, 80 amps is way too much. 60 ok, but not 80. That's for the bms to not trip every ride on one of the packs. To make your battery run cool and happy, you need to limit them to more like 20 amps each, if 40 is the limit. Happily, you can really haul some ass with 72v 40 amps.

As stated above, verify voltages before making a connection. Have some way to do that, besides just a green light on a charger.

Tape on the connections will help prevent a wire from having a disconnect, that results in a hard ride for the remaining pack. I've smoked a battery by having the other one disconnect. (without bms in that case).

Dogman and Sonder

Thanks. Alright one more reply please Lets keep it back to basics simple KISS. From the start I almost had my goal with a single battery and a little shitty 72v 40a probably 12 fet controller. 3000w motor The 2.5 in hookworm tires would sink in the soft sugar sand and me giving it
more power the controller would get hot and shut down. Let it cool plug it in again and away you go. So I thought build a fat tire and get a bigger controller.Well after one controller I bought 60a arrived DOA. I bought an 80a 24 fet. It is huge. So now I thought how about running two
batteries in parallel with the bigger controller. But after some thought I realized I got a 40a bms in each. Well all I want to do is get some more amps with the current batteries. These are identical just a couple of months apart same cell same everything and using high quality wire and anderson connectors. So I'm not planning on a dirt bike race I just want some more amps when I need to pull thru some sand. I am connecting
these two batteries with a 80 a anderson plug and 8 ga wire. All splices soldered / welded / shrink wrap.Lastly your advise is a $9.99 insurance
policy. I have one digital volt meter on handlebars spliced into the controller main battery wires. I will buy another one 10 bucks amazon
and install both to the bateries not controller. Very good point trip one bms and not know it and you can fry a 599.00 battery. I just fired
up the controller / motor. Hall phase wires matched just like last the last Mxus v 3000 motor I bought even though the controllers were doifferent. How lucky is that. Let me know what you think. I am waiting on a Blackburn Outback Fat Bike Rack to mount my second battery.
It will go together shortly. I will oreer another volt gauge and have two on the handlebars Great point Now I can monitor both batteries right on the handlebars. I can also see any volt sag if one is pulling more than the other. Thank you again. Jack

Well, at some point, with the tires well dug into sand, you will pull the 80 amps for a spike. The main thing you want to avoid, is one bms popping, leaving you pulling 40 amps more continuously from just one of the packs.

I'm not sure how you know both are pulling, short of having a watt meter on each one, before the parallel.

One thing that could help, is tone down the 80 amps controller some, but that just puts you back into not enough power when you dig it in.

I do know what you are up against, Ive melted more motors by being dug into sand or other soft dirt than climbing hills, by far. I would like to do a fat tadpole, just for riding the sandy dry washes out here. Currently I have to just cross the sand, and ride the ridgetops. The sandy bottom will melt me down a motor.

Popping controllers all the time, that is sometimes a symptom of too much load, on too fast a winding, in too big a wheel. Maybe you just need scooter size motors and tires?

At this point we will find out soon. I think the motor can handle it. Gut your right we will see if A pulls More than B etc. I tried to eliminate that possibility. One battery is about 5 mos old and has probably 75 charges. The other one is new. Here is the controller it is huge and heavy

https://www.aliexpress.com/item/72v-84v-96v-3000w-Vector-Controller-3kw-24-MOS-Electric-Car-Controller-Electric-Hub-Motor-Wheel/32798248675.html?spm=2114.13010608.0.0.8NrK98

I just rewired both my battery connectors, controller, battery lea
ds and Y split. All brand new stuff. All 8 ga fine stranded same 80a connectors same length same solder weld
job with shrink tubes. That should help. Here is the controller it is huge and heavy

https://www.aliexpress.com/item/72v-84v-96v-3000w-Vector-Controller-3kw-24-MOS-Electric-Car-Controller-Electric-Hub-Motor-Wheel/32798248675.html?spm=2114.13010608.0.0.8NrK98


Here are the connectors I used I replaced all of them . I used only the quick disconnect anderson type plugs the wires
connecting each other were cut spliced and inline braided . then soldered / welded then double shrink tube.
Super solid. If there is an issue it won;t be a connection problem. jack

https://www.amazon.com/gp/product/B0027UTJ8U/ref=oh_aui_detailpage_o03_s00?ie=UTF8&psc=1

I see you have a three speed switch plug on the controller, if it pops the bms on you batteries with too much amps, use the lower setting, or less throttle.

The most likely time to have a pack cut off leaving one pack bearing the whole load is when one is empty and the other just has a bit left. It would then only see 40 amps, then it would shut off. But at the end of the discharge is no time to be hitting your pack with 40 amps.

Slow down a lot, when you see very low voltage on your voltmeter. Get one on the bike if you don't have one.

dogman dan

With your previous suggestion I ordered a second voltmeter and I am mounting both on the handle bars one for each battery.

I have used these before they are great cheap and put out a cool blue neon like digital light that looks hi tech. I recommend them

Check it out.

https://www.amazon.com/gp/product/B00NZV7K3G/ref=oh_aui_detailpage_o01_s00?ie=UTF8&psc=1

Thanks

Both batteries should be very close to the same, even under load. The voltameter will only tell you which battery is stronger in the few seconds after a heavy load.

What would be more useful would be ammeters on both batteries.

Try to insure that both battery wires are of the same length up to the Y parallel connection. Reason being, a longer wire will have more resistance, therefore the Batt with the longer discharge wires will discharge slower than the other, with obvious results. Also, some may disagree with me, but I am pretty sure both aforementioned volt meters are going to always read the same volts unless the batts are disconnected from each other. This is because they are basically attached to the same wire while batteries are connected in parallel.

Yes, that's why I said two watt meters, Not two voltmeters.

Thanks The cable lengths are the same and the meters are wired directly to the batteries before the Y connection. It should
give me a correct volt reading correct ? Dogman a watt meter ? I got to look into that. Above my pay grade for now. But for now we are going to
fire up what I got this weekend. I will let you guys know. Thanks Jack

That 80 amp huge controller got to be shit hesitant at start, motor makes wierd start noice motor gets hot and not pushing it

Well guy I fired it up. 1st issue seems throttle a slow and hesitant at start of then power up smooth .
Throttle the whole time The motor seemed to make a weird wind up or grind noise
slow to kick in it hesitant and hesitant throttle response all thru range. Now I does not work DEAD I swapped my luna 40 amp wired it up hit throttle and motor and everything good.

I got a luna cycle 40a 72v controller and it is smooth and peppy off the line just lack deep pulling power..I got this new one coming from Luna cycle 60 amp 72v bloetooth programable 99.99. I will fit if the wiring the same I can handle that.

Hare is the link https://lunacycle.com/60-amp-36-72v-ebike-bluetooth-programmable-controller/#ProductReviews

Should this cure my problem ?





.I got this from Luna cycle 60 amp 72 v bloetooth programable 99.99. I will fit if the wiring the same I can handle that.

Hare is the link https://lunacycle.com/60-amp-36-72v-ebike-bluetooth-programmable-controller/#ProductReviews

THIS IS MY SECOND SHIT CONTROLLER ON THIS PROJECT BIKE > i STILL WANT TO TAKE OFF THE BATTERY y hARNESS FROM THE POS

and put it on the new 60 amp coming from luna and put in on the new battery mains on this controller
Another issue is the motor got hot for my test riding I was not pushing it but it was way hot for what I was just testing.The motor if fine
I swapped a luna 40a 72v controller and it fired up fine. Maybe this 60a luna 72v is what I need.

Well if the 60 amp controller pops the bms we know what's up.

tjgaryiv60 said:

With your previous suggestion I ordered a second voltmeter and I am mounting both on the handle bars one for each battery.

Click to expand...

Don't bother; they will be identical, or so close to identical that it won't be informative.

bill von

Your right both volt meters read the same. One was 79.8 one was 80.1 before plugging them into controller in parallel then both read 80.1.
I thought with the connection right out of the batteries I would get the volt readings before the Y harness in parallel. Wrong.

Hopefully the new Luna 60a 72v controller is what I need. The 40a I have works great smooth but not quiet enough power I want on the
new fat tire bike.

Can I still try to run both battery packs in parallel to this ? 72v 20ah 40a continuous bms ?

Thanks

Yes.

It sounds like with the other controller, you had the thing running with the wrong phase order. That makes the motor and controller run hot. This will blow a controller, and it sounds like it did.

You usually can't just match up wire colors and run it. You have to find the right order, or use a self learning feature of the controller.

Power up the motor enough, it will sound very different. It will wail. But it should not have been hesitant.

Thanks

I'm done with chinese controllers. Next one a kelly. I got a luna cyclone 60a coming and currently running the bike
on a luna 40a controller. The bike needs a little more power thru deep sand. Maybe this will do it. Otherwise Kelly.
We will see. I got the blutooth app installed on phone. I read the phase amps should be toned down for smoother throttle response.
Any suggestions on phase amps. I was going to go 33% lower and try that. Jack

Deep sand ,mud and steephills those are the things that melt the motor. When a motor lugs it's making Heat. Just like when you're trying to use too big of a bit in a drill and it starts wrimmmmeeee. That's heat. The motor likes to run fast. Ask Dogman how to melt a motor.