Acceleration Slows Down

GEM Forum
#1

Hello everyone. I recently purchased a 2017 Gem E6. I had to make a few mods to it, below are the modifications:

  1. 14" Wheels sitting 23.5" tall
  2. 8 x 12V LifePO4 Batteries 100 AMP. 2p4s ~ 48V 200 Amps
  3. Ride4Fun SpeedKit 1, to my understanding it is a gear box upgrade

Everything was running fine for the first 2 months after the upgrades, the AMP draw to my batteries would reach about 300 AMPS during acceleration and top off at around 120 AMPS when reaching about 34-35 mph.

Over the past few days, when the batteries would get around 65% SOC I noticed during acceleration my AMP draw would only get to 180 AMPS thus slowing down acceleration. Top speed would be the same around 34-35 MPH the only issue is during acceleration the Gem isnt coming off the line as quick as it used to. During the first 2 months my batteries could be 50% SOC and I still would come off the line quick.

I dont know if this is a battery issue or motor controller issue. During the past week I did have a stereo with 4 speakers installed and I also made sure my tires were 35 psi, where before they were around 28 PSI. I dont think any of those would cause this issue, but that is why I am writing this topic.

Any help would be most appreciated.

PS: I know the LifePO4 batteries are not the best and most ideal, but it was hard for me to find a charger to satisfy batteries like the Chevy Bolt or Tesla batteries.

1 Like
#2

Sounds like battery issue as it works with full battery. Other issues should trigger error codes. I would not trust soc with lifepo4 batteries.

#3

Not that I am an expert, but I have some Q’s-
It sounds like you did the install- Yes?
Which batteries did you install?
What are you using for a charger?
Are you using a BMS?
Is it possible that the batteries got out of balance?
Have you measured them lately or are you monitoring that on your BMS?

It kinda sounds like you lost half of your pack. Have you looked for a loose/burned connection or cable?

If that checks out ok-
After a good run, how hot did your motor typically get?
Does it pass the sniff test?

1 Like
#4

Good idea. With 4 discrete battery systems it’s bound to happen. Top balance them to see if it helps.

#5

I have a victron battery shunt which is what i use to determine SOC

#6

Yes, I did the install. I am pretty familiar with DC battery equipment, not so much with motor controllers. The only thing I did myself was the battery setup.

These are the batteries, I put 8 of these: TOBattery 12V 100AH LiFePO4 Lithium Iron Phosphate Deep Cycle Battery 24/36/48V | eBay

This is the charger I am using: https://www.amazon.com/NOCO-GX4820-Ultrasafe-Industrial-Battery/dp/B01FRQQKAK/ref=sr_1_3?crid=GEUDBA4BPBV1&dchild=1&keywords=noco+48v+charger&qid=1614865089&sprefix=noco+48v%2Caps%2C163&sr=8-3

Each 12v battery has its own BMS

I need to check the voltage of each battery, that is a possibility. I will check on that today.

My Victron Battery shunt show a voltage of around 56.8v on full charge, these can reach to 14.4 per pack, so maybe the batteries need to be balanced.

I need to measure the temperature of the motor, today down in Sunny South Florida the weather is actually beautiful, maybe I ought to drive around and see if I get the same results.

#7

Excuse some of the rambling. You probably already know most of this. I’m just going through this with the other voices in my head. I often find that freestyle discussion sometimes brings out good ideas.

Each 12v battery has its own BMS

That is what I was guessing. Makes it simple to wire up, but you have no real control on what is going on in there. Hopefully it is not working these batteries too hard to get it’s rated numbers at the expense of overall life. I’ve seen a few vids where a guy cuts these things apart to see how they are made and he rates their components. They are quite educational. Some of them have very underrated wires between the BMS and the terminals you see. I’ll see if I can find one of this battery. He is more of a off-grid/RV view, but still informative.

I need to check the voltage of each battery, that is a possibility. I will check on that today.

That is exactly where I was going with the suggestion.
I think you need to do a individual battery check at full charge AND low charge. (65% SOC on your meter or whenever you note the loss of accel). Take notes. There are alot of numbers to remember. You might even find one battery shut down and another getting ready to.

I could see the individual BMS taking care of it’s own internals, but it has no idea of the state of the surrounding cells. Depending on how you wired them up makes for interesting scenarios.

Thinking that the internal BMS disconnects the battery on LowV, if one drops out early it passes duties to the other in in the pair. I don’t think the remaining battery will do 300a by itself.

It would be interesting to see what the BMS does on HighV.
Does it shunt? Does it just bypass current through?
Are all of your batteries getting a full charge?
Perhaps a 12v/4 bank charger would work better in this situation?

#8

Sorry for the late reply folks. It was quite some time until the slow acceleration happened. And it just happened again today. Weird thing is that it happened today when battery was at 95% SOC, continued to happen through the entire day even when the battery was fully charged after my initial ride. The checked the voltage in each individual battery and all of them voltages read the same 13.29v. At fully charged all the voltages also read the same. So it definitely does not look like a battery issue at all now.

I am not wondering if this is a gear issue. I have no idea how many gears are on the car, I did a gear replacement at the Gem Shop for Ride4Fun’s Speed kit 1. The only reason why I think it might be a gear issue is because top speed is still attained at around 34-35 mph, it just takes some time to get there. It seems like it never starts in first gear. Anyone have any gear issues?

#9

Test voltage under load at the controller.
Not much else can go wrong with sevcon and a.c. motor.
Hard for me to see how parallel bms can work. You may need to monitor current on each parallel bank to make sure they are sharing the load.

#10

I have a victron battery shunt and as I accelerate the combined voltage sits around 48v. If there was a bad battery I would expect voltage to be lower on load. I dont think it would be the controller or motor. Can it be the gearbox?

#11

I disagree, it’s the configuration that I find suspect. I got 16 280ah lifepo4 delivered for $2400. Been working great. In a new gem I would suggest 18 of them. 200ah is plenty if it breaks the budget.

#12

I do have LifePO4 batteries. I literally checked every battery’s voltage. I have a total of 8 - 12v 100ah batteries. I put 2 in parallel and 4 in series. These batteries were purchased brand new about 3 months ago.

#13

As I understand it the are black box batteries for replacing volt lead batteries. I have yet to here a long term success story in a Gem. In fairness the only ones I know of are in 72v configuration. 48v should be better in theory.
If the discrete batteries can’t communicate, I don’t see how they can share the load properly.
I may be wrong, but the only way to know for sure is to substitute for a known good battery system.

Imho
@LithiumGods

#14

Yes, the black box batteries. From my understanding, when running 2 batteries in parallel they just maintain the same voltage. If 1 battery is bad, the good battery will attempt to charge the bad and eventually cause issues and I will definitely see a drop in voltage. Each box has it’s own BMS to make sure the cells within do not get damaged. It was running perfectly fine for the first 2 months. I had torque and speed, now I am just getting speed and NO torque.

#15

Unfortunately with lifepo4 voltage means very little. This is goof for function, but bad for trouble shooting of checking soc.
Do the have any data port or monitor system to see what they are doing. 32 cells are difficult to keep in sync.

#16

Not one that I can access, each set of 4 has their own internal BMS

#17

put a volt meter on each battery and run it. See if one is sagging on acceleration.

#18

With two ammeters you may be able to see an issue with current sharing.
Connect so each METER reads only one parallel battery . Not easy to do, but the only thing I can think of.

#19

That might works. But as two are connected post to post, it won’t isolate which one of the two.

I would be interested to see the actual layout of the series parallel connections. If it were me, I would connect all the p connections and all the s connections. Equal length of course. Lots of cables, but I believe it’s needed to have a shot at this working.

#20

Let me try and find multiple volt meters with long cables. Just out of curiosity why are we ruling out the gear box? It literally feels I am totally skipping the first gear and jumping right into 2nd.