1991 ezgo electric - slow take off

[hylomatt]

I have a 1991 electric marathon with 6 new crown batteries. Take off from stand still is a crawl up to speed. I've tested the pack voltage (38.7v), measured the resistance on the battery cables (0.0 ohms), measured the resistance of the accelerator potentiometer (0 - 6370 ohms). I've also made sure that the brakes are not dragging.

I can't figure out why its acceleration is so slow. There's seemingly no torque to offer when the pedal is pressed. It reaches 11-13 mph but you don't feel any "pull" until about 6 mph is achieved.

If anyone can give me some advice on what to check or what might be causing this, I would appreciate it.

[Newbuggyboy]

My 91 marathon accelerates off the line and the front lifts. The wheels don't come up off the ground, but in is exhilarating. My potentiometer goes to just under @6,800. I think I saw somewhere that there is an adjustment for the acceleration curve. I'll try and dig it up. Mine runs so nice I don't want to touch anything. Maybe someone else know about that acceleration curve.

PS My pack voltage starts out somewhat higher than yours. My pack is new.

Edit note for those that tease me about street outlaws. I'll need to test for how many Gs the cart puts on me off the line. I'm sure Big Cheif would be proud.

[hylomatt]

My pack is new as well. It's a new set of 6 crowns, CR-205s. Less than 2 weeks old. I got rid of the old, heavy ezgo textron charger and got a nice, new, fancy modz smart charger so these batteries don't get overcharged. I let them charge for 24 hours before I put any load on them and I've been checking their water levels before and after charges, just to be safe.

I should say they are 2 weeks old but we've been gone for a week so there's less than a week of usage on these batteries yet. Maybe used it 6x since we got them.

[JohnnieB]

New batteries only have about 75% of their rated capacity until the are broken in, which takes two dozen charge cycles or more. So you 205AH batteries are effectively 154AH and climbing.

Also, it is winter and lower temperatures lower battery capacity and performance. (See attached chart).

As for charging, I've attache a SoC chart for Crown Batteries.

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What make/model controller?

What height are the tires? (stock is 18")

[hylomatt]

@JohnnieB, thanks for the images. I live in Florida so at the moment that puts me between 80%-100% capacity. It's around 50ish in the mornings but maybe 70's in the middle of the day.

As for the SoC chart, when it says 12 hours at rest, does that mean off the fancy tender charger? Or on the charger but charged and unused for 12 hours?

I'm not sure about the controller, I've never removed it from the cart and I don't see any visible identification on it. I've tested via the screw terminals and removed the 3 push-on connections, cleaned, and reconnected them. Tested voltage and ohms on it as well.

The tire size is stock 18x8.5-8.

[JohnnieB]

Originally Posted by hylomatt

@JohnnieB, thanks for the images.
1. I live in Florida so at the moment that puts me between 80%-100% capacity. It's around 50ish in the mornings but maybe 70's in the middle of the day.

2. As for the SoC chart, when it says 12 hours at rest, does that mean off the fancy tender charger? Or on the charger but charged and unused for 12 hours?

3. I'm not sure about the controller, I've never removed it from the cart and I don't see any visible identification on it. I've tested via the screw terminals and removed the 3 push-on connections, cleaned, and reconnected them. Tested voltage and ohms on it as well.

4. The tire size is stock 18x8.5-8.

1. Golf cart batteries have a huge thermal mass, so the electrolyte temperature lag air temperature by several hours. The overnight low is probably the best yardstick to use for most of the next day.

2. About 12 hours off the charger and the cart not used.

3. Sounds like a Curtis 1204.

4. Tires taller than stock reduce the torque where rubber meets road and slows acceleration. Your stock hieight tire shouldn't cause abnormal acceleration.

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Connect a DVM between the battery pack's main negative and main positive terminals and monitor pack voltage while acceleration from a standstill. What is the lowest it drops to?

[hylomatt]

@JohnnieB, thanks for the info.

1) got it.

2) I'll test that. I've had it off of the battery tender for maybe 6 hours now (and up on jack stands, replacing the brake cables). I'll wait longer and test. it's sitting at 38.4v atm)

3) I did find a sticker on the bottom. It says Curtis but the model number is unreadable. You're probably correct.

4) Stock tires so I should have stock acceleration.

When I first got this cart, it was much peppier when you hit the accelerator. Then the batteries were severly abused and ignored by letting them go dry for an unknown amount of time, some frame-welding and battery tray repair happened then finally new batteries very recently.

So the batteries aren't "broken in" to their max potential. Is this the reason it just crawls up to speed when you first hit the accelerator pedal?

[hylomatt]

After sitting overnight, the battery bank reads at 38.1v

[JohnnieB]

Originally Posted by hylomatt

So the batteries aren't "broken in" to their max potential. Is this the reason it just crawls up to speed when you first hit the accelerator pedal?

Probably a contributing factor, as is the cool temperatures, but there are likely other factors.

A poor electrical connection comes to mind. Excessive resistance in any of the cables, connections or contacts in the high current loop will drop the voltage applied to the motor and the more amps being drawn, the greater the voltage drop.

It's often stated that volts are speed and amps are torque, but in reality they are not separate entities because volts and amps are interdependent. Volts are the force that pushes amps through the motor's impedance (resistance) and if you lower the voltage, fewer amps flow and less torque is produced.

The effects of battery newness and sluggish chemical reactions due to temperature will be seen in how much the voltage the individual battery's drop when accelerating from a standstill or going up a hill (The sum of the individual battery voltages will be what the pack voltage ought to be with healthy cables and connections). The effects of a bad cable, connection or contact will be seen in the voltage drop between the controller's M- and B+ terminals during the same.

Another indicator of a bad cable, connection or contact is heat. If anything is getting hot, it needs to be repaired or replaced.

[hylomatt]

I just finished removing all 4 of the motor connections, wire-brushing them (with a hand drill) until they were shiny, used a little dielectric grease on the post and the ring terminal and locked them down again. I did the same to each of the battery terminals and to the 4 post and ring terminal connections on the back of the F&R switch.

The connections on the controller don't budge so I left those alone for now.

I get full battery pack voltage on the M- and B- terminals on the controller when I sneak the gas pedal on and it drops to about .1v all the way down. During my testing the battery pack was dropped to 37.1v but I see every bit of it at M- and B- terminals nonetheless. It's on the charger now.

Any other ideas?