Wire gauge bottle neck?

Thread Starter

1quickquestion1

Joined Oct 13, 2017
22
I recently built an electric skateboard and it gets pretty low mileage per charge, so I built a battery to replace the original one. This battery has 3x the current (6000 mah) and the same voltage at 25.5. The way I constructed the battery is by wiring 2 pack containers of 3.7v cell batteries together in series. Each of the packs use quite thin wires but the packs were designed to be used for this battery (but not necessarily in series). When I run the skateboard on its own it works well, but once I apply weight and strain, the motor briefly starts working and then shuts off, and the battery indicator says the battery is dead. Once I get off the board and run it without load it works again and the battery indicator shows full again? I have no idea why this is happening so any thoughts would be much appreciated, thanks.
 

AlbertHall

Joined Jun 4, 2014
12,071
The new battery may have a higher capacity but be unable to supply high current.
What was the original battery and what replacements did you use?
 

Thread Starter

1quickquestion1

Joined Oct 13, 2017
22
The original battery was the same voltage ratting but different current, so the original was 25.5v 2000mah and the one I made is 25.5v 6400mah. But I noticed the gauge of wire on the original battery is much thicker than mine.
 

AlbertHall

Joined Jun 4, 2014
12,071
But what type number are the two batteries so we can look up the data on them. The difference in wire gauge sounds like it supports what I said earlier. Because a battery has a larger capacity does not necessarily mean that it can supply a larger current.
 

WBahn

Joined Mar 31, 2012
26,416
The original battery was the same voltage ratting but different current, so the original was 25.5v 2000mah and the one I made is 25.5v 6400mah. But I noticed the gauge of wire on the original battery is much thicker than mine.
mah is NOT a current, it is a charge rating (the total charge the battery can deliver under nominal conditions).

The current rating is VERY different from these and the two are largely unrelated.

The heavier wire supports the probability that the lower capacity battery is designed to be able to deliver substantially higher currents (for a consequently much smaller amount of time).
 

Thread Starter

1quickquestion1

Joined Oct 13, 2017
22
I am using "BRC 18650 6800mah 3.7v li-ion batteries" in series (7 of them) using 4 battery holders which each carry 2 batteries (using thin wires which read AWM 1007). The battery I am replacing is an "HYY-&S1P lithium ion battery 25.2v 2000mah inner cell domestic" battery pack (which uses 14 awg wires).

I was under the impression that mah was related to current? Would the current reading then be only found on the esc or the battery pack?
 

WBahn

Joined Mar 31, 2012
26,416
1 mAh is the amount of charge that passes a point when 1 mA of current flows for 1 hour, which is exactly the same amount of charge that passes that point when 1 A of current flows for 1 millihour (also known as 3.6 seconds) or 1 μA of current for 1000 hours (also known as nearly 42 days).

This is similar to putting a container of water up on a platform and rigging up a showerhead. When you take a shower you are primarily concerned with the flow rate of water (gallons/hour, for instance, or gph). So say that you have settings for 100 gph, 500 gph, and 1000 gph. Your tank, to make things easy, might be sized in gph-minutes. So if the tank were 1000 gph-minutes then it could support a flow of 100 gph for 10 minutes, 500 gph for 2 minutes, or 1000 gph for 1 minutes.

But, fundamentally, gph-minutes is a measure of volume -- to be specific, 1 gallon = 60 gph-minutes.

Similarly, 1 mA-hour is fundamentally a measure of charge. Since an ampere is 1 C/s (coulomb/second), 1 mA-hour is the same as exactly 3.6 coulombs of charge.
 

LesJones

Joined Jan 8, 2017
3,745
Using battery holders is not suitable for high currents as they normally use spring contacts. Apart from the problem you are seeing you should also consider using a battery protection circuit which will balance the charging so that the lower capacity cells do not get overcharged. they also prevents the lower capacity cells being reverse charged when powering a load. For a high current application (As yours is.) it is normal to make the battery pack by spot welding metal strips between the cells. Making lithium ion battery packs is not as simple as making NiCd or NiMh packs.

Les.
 

Alec_t

Joined Sep 17, 2013
12,889
I was under the impression that mah was related to current?
It is related, but it isn't the same thing. mah means ma times h (where h is the time in hours).
Drawing current from a battery is like drinking beer from a glass through a straw. For a given flow rate through the straw (think ma), a full big glass (think mah) takes longer to empty (think h) than a full small glass.
 

MrAl

Joined Jun 17, 2014
9,151
Hi,

Thin wires drop more voltage than thick wires. They can also get hot and that makes them drop even more voltage. For low voltage applications like this that could make a big difference.

It's not just the thickness though, it's also the run length. Longer wires drop more voltage while short wires drop less voltage so you can check that too.

Li-ion batteries have to be charged much more carefully so you better look into this more deeply.

Li-ion batteries also have a maximum discharge current rating, and an internal resistance. That means not all Li-ion batteries will work for every application. For example, i had been using run of the mill 18650 cells for quite a long time but when i purchased a high powered LED light (somewhere around 70 watts) i had to buy high current Li-ion cells which were more expensive but could deliver the power the light needed. The other cells i had could not handle that much current.

Buying Li-ion cells is not the same as buying AA alkaline batteries. AA alkalines are almost all pretty much the same while Li-ion batteries have different characteristics. You have to get the right battery when dealing with higher currents. You can tell which is right by looking at the specs for the battery. If you post the make/model we can look it up here.
 
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