how do I find out the current capacity of my batteries?

[mrklaw]

Had a good run with my lipo pack in the park, and I want to be able to check whats left in it, so I can start to get a feel for how long I can run before hitting the voltage limits. I have an Ansmann xbase deluxe which gives me all kinds of volts/mah while charging, but doens't seem to give me any readings if I just hook the battery up.

is there something (cheap) that'll let me do that? and for lipos/NiMh if possible

[Garry]

I don't know about capacity, but you can put a multimeter over the terminals after a run and see what the pack voltage is.

[TiM!]

Sadly there is no way of telling how many mah are left inside the pack.

The charger only tells you how many its put in by the current it's set too, and the time its been on charge.

Edited May 23, 2009 by TiM!

[mrklaw]

if I charge it up at 1C, it beeps when its finished, so will that give me any indication of whats been used?

eg if it was half drained, will it take 30 mins to charge up again?

[Garry]

Yep, but you'll get an accurate reading as the charger screen tells you the capacity in mAh put back in.

So a 1200mah pack putting 600mah back in after a 20min run means you'll get around 35mins before needs recharging.

[mrklaw]

hmm

It just finished after 100 minutes, and it says its pumped 2600mah into it. its a 4900mah pack, so just over half used. Pretty good. Few more like that and I'll get a feel for it.

but why did it take 100 minutes to only fill up 50% of the pack? Surely at 1C it should take an hour to charge from empty? whether that means it should take 30 mins from half empty or an hour - it shouldn't take 1 hour and 40 minutes should it?

its set to 7.4v, 4900mah. The charger should charge at 1C automatically, and its a 5A max charger

[Perko]

Had a good run with my lipo pack in the park, and I want to be able to check whats left in it, so I can start to get a feel for how long I can run before hitting the voltage limits. I have an Ansmann xbase deluxe which gives me all kinds of volts/mah while charging, but doens't seem to give me any readings if I just hook the battery up.

is there something (cheap) that'll let me do that? and for lipos/NiMh if possible

I use the Fusion smart guard to check capacity of lipo's works pretty well, gives a percentge left rather than actual mAh:

http://store.modelpower.co.uk/fusion-smart...ecker-103-p.asp

[bb2120]

This is how I've always found out how much capacity my packs have in them, based on the fact that the discharge of LiPo is quite linear.

For a 2s pack:

8.4V is fully charged

7.4V is 50% charged aka 'nominal voltage'

6.4V is <10% charged

Based on this, you can measure the voltage across the pack and get a reasonably accuracy value for capacity.

For example, a 2s pack at 7.9V is roughly 75% charged.

This is essentially how battery meters on phones/iPods work, although different batteries have subtly different discharge curves.

Edited May 24, 2009 by bb2120