DIY Electric Car Forums banner

Initial discharge run and understanding my LiPO4 batteries.

893 Views 4 Replies 2 Participants Last post by  telpochyaotl
Hello All, I'm new to this forum, this is my first post. I appreciate all the help in advance.

My DIY car is a 1994 (mexican model) VW Bug. I have a 72v brushed PM DC motor, a Kelly KDZ72401 controller, and the current source of my sorrow, a 100 AH 76.8v battery pack made with 120 20 AH 3.2v cells arranged as 5p25s.

Honestly, I didn't know what I was doing (still trying to figure out) when I bought these as a kit from some online seller. I finally put it together for a first discharge run, after which I took the battery apart for bottom balancing.

On this first run yesterday, I took the car out for what ended up being a very short (7 mile) drive, with the factory charge. All batteries were 3.2x or 3.19v when I started. The thing ran fine for 5 miles or so, but once it hit the 70v it declined pretty fast and I had the controller programmed to stop at 60v.

While using it, i did get up to 50 mph for a short period of time, then stayed for a minute or so @ 40 mph and most of the driving was under that. The AMP meter was briefly, for a second at a time or so above 100 amps, and while cruising between 50 - 90 amps.

When I took the battery pack apart, i tested every battery, none seem dead, but a good bunch look swollen, these are between 2.40 and 2.43 volts and the other ones are between 2.6 up to 2.9 volts.

At this point I have 2 theories, one of them is hope, the other one is more factual.

1 - the factory charge was not 'good enough', once I bottom balance these and charge them well, the pack will perform as I expected and give me twenty some miles per charge, hopefully more.

2 - I totally don't understand how these batteries are supposed to perform and need to figure it out or I will fry these and the next pack i end up making.

The specs for these batteries are:

NOMINAL/MINIMUM CAPACITY: 20Ah @ 1/3C Discharge
STANDARD DISCHARGE CURRENT: 6.7A (1/3C)
MAX DISCHARGE CURRENT (continuous): 100A (5C)
MAX DISCHARGE CURRENT (<10 Seconds): 160A (8C)
STANDARD CHARGE CURRENT: 6.7A (1/3C)
MAX CHARGE CURRENT (continuous): 100A (5C)
DISCHARGE CUT-OFF VOLTAGE: 2.0
NOMINAL VOLTAGE: 3.2
SUGGESTED MAX CHARGE VOLTAGE: 3.65
CYCLE LIFE AT 100A (80%): 500
CYCLE LIFE AT STANDARD (80%): 4000
SELF DISCHARGE RATE (30 days): 4-6%
INTERNAL RESISTANCE/IMPEDANCE (milliohms): 2
ENERGY TYPE: 3V-3.2V-3.65V, 20Ah EV, Lithium, LiFePO4, LFP Aluminum Shell Battery

Obviously I don't understand how the AH and C work. In my simple mind, if I made a 100 AH pack, i could get 100 amps for an hour, I don't understand why each of the cells is 20Ah @ 1/3C.

Any help figuring out what happened and will happen is greatly appreciated. Right now I know I paid too much for these batteries, and I'm unsure if they are well suited for this small VW conversion, though the supplier claimed these were great.

Thanks a lot, sorry for the naiveness..
1 - 5 of 5 Posts

· Registered
Joined
·
281 Posts
When you paralleled the cells the ah add together. So the 1/3c would be figured on 100ah.

I would say at 3.2volts is not fully charged. I have never run my pack that low since bottom balance.


Alvin
 

· Registered
Joined
·
3 Posts
Discussion Starter · #3 ·
When you paralleled the cells the ah add together. So the 1/3c would be figured on 100ah.

I would say at 3.2volts is not fully charged. I have never run my pack that low since bottom balance.


Alvin
So, 1/3c would mean that my 100ah would give me 33.33 amps continuously for an hour? I'm curious why call it 100ah then, isn't this supposed to mean 100 amps for an hour?

This initial discharge was for bottom balancing, I didn't have a better thing to discharge them than the motor so I decided to use it. Right now I'm trying to balance them all to 2.5, and then I'll rebuild the pack and charge them all.

Thanks a lot for your response!
 
1 - 5 of 5 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top