Why you should have 4cell mod?

[jasonip]

Did you ever try to measure the volt. of the batt. after the run out of batt. and stop? You will find that it reads ~3.80v , that means the electronics have a cut off volt. at this level.

But when you look deeper for the discharge characteristic of NiMH ( http://www.gpbatteries.com.hk/pic/GP10AAAHR_DS.pdf ), the original 3cell means that each cell is ~1.26v, which is only used 25% or maybe fewer of the whole batt. life cycle.

How about charging? The stock charger gives a 750-700mA charging for 2 mins. You can calculate that it gives 725mA*120s = 24mah . In comparing with the capacity of the cell 120mAh, it is only 20% of the batt. It seems that my calculation before is reasonable. (We have to notice that the batt. is charging at ~6C, which is much higher than the advice fast charge current, it will harm the batt. if charge too long)

So how much current is drawn by the car? You can estimate that 24mAh for 5mins run, it cost ~290mA. When you look at the standard SH-030SA motor ( http://www.mabuchi-motor.co.jp/cgi-b...AT_ID=sh_030sa ), max efficiency is 0.22A, therefore ~290mA current drawn estimation is reasonable.

When making a 4cell mod, it makes each cell can be discharged to ~0.95v before the car stopped, which can use up the whole cycle of the batt. But the charging time need to be increased, otherwise the playing time won¡¦t increase much (people response that the playing time increase, becoz when volt. increase, fewer current will be needed¡K¡Kbut this still didn¡¦t use the whole batt. cycle)

Charging time should be increased to 4 times the original (for safe and not overcharging). But charging at 6C for 8 mins will definitely harm the batt. , I suggest to use an appropriate charger rather than the stock one. (The charging current of the stock one will drop from 750mA to ~6XXmA, this data is referring on 3cell charging. When using 4cell, the charging current must be lower than that. I haven¡¦t try 4cell yet, but some post say that charge for 10min¡K¡K. )

NOTICE: In here, all data are measured with original 3cell spec 1 car. And I don¡¦t guarantee all the information in here is error free. So¡K¡K.Take your own risk!!!!!


------ Plz give me some feedback if you interested in this, or point out my mistake. All are welcome to share what you know.
(wow, very long article, thx for reading)

[jasonip]

Originally posted by jasonip
(people response that the playing time increase, becoz when volt. increase, fewer current will be needed¡K¡Kbut this still didn¡¦t use the whole batt. cycle)

I say that becoz when running at top speed, higher volt. would increase the speed, which increase the impedence of the motor, so lower the current, and thus for longer run time.

When both 3cell & 4cell car are running at the same speed, the power dissipation should be the same.

(all in all, 24mAh in 4cell must contain more power than 24mAh in 3cell)