Hey guys,

I need help. I have an accu in my transmitter which seems to be empty and full again way too soon every time i use/charge it.. is the battery too old, or might it be something else...?

specs: accu: Robbe NCmh 9,6 V, 750 mah, 8 cells. It says to charge for 14-16h at 70 mAh, max 1,5 A.
charger; IMAX B6. I've tried charging at 0,7 A and 1,5 A, and it indicates "FULL" after about 20 minutes. doesn't take more than 20 minutes of use to empty it again, though..

any tips? would be much appriciated! thanks!

You have a small capacity (750Ma) Nickel battery pack.

The instruction you have are for a SLOW charge generally using a TX wall wart kind of charger.
The B6 is a delta-V peak charger and it's terminating prematurely or the pack is shot and won't take a charge. You are also using too high a charge rate.
Whether that's a problem with the charger or pack remains to be seen. OTOH the pack never was and never will be very good.

If you want to try to figure it out get an appropriate slow charger and use as directed.
Or turn the charge rate down to 0.1A (100mA). and charge for ~10 hours or until the pack gets warm (NOT HOT!). Check it once in awhile. The B6 should not terminate at that low setting.
You generally don't want to go too fast with those kind of cells.
You are using 700 and 1500mA charge rates which is not good for those kind of small packs. There may be a diode inside the TX that interferes with charging with a peak charger.

IMO-get a better pack. It's not hard to get LSD type NiMh packs now of 2000mA like Sony Eneloops. These will last for hours of flying and not lose their charge sitting on the shelf.

People will suggest a 3S lipo. You can do that but the first time you forget to turn off the TX and come back to a dead lipo you'll regret the choice.

thnks!
I was thinking about changing it indeed, but I'll try charging at 0.1 A for a while first.

Most TX pull something like ~200mA. So if that pack is freshly charged and top notch it's maybe good for 3 hours. It'll also lose 10% of a charge in the first 24 hours (normal) and then ~ 1%/day, worse if it's in poor shape.

Some of my early TX packs were 500mA NiCad....poor. I had a couple new 1500mA Spektrum packs. Should have been OK but they must have been lousy cells as they weren't worth much either. My LSD packs are great.

LSD is WAY better. More capacity and significantly improved self discharge.

On all NIxx packs, you will get a full charge by charging at 0.1C for 15 to 16 hours. It will not hurt the battery if you leave it on for a bit longer at that low rate. You can NOT use a peak detecting charger for this forming charge, just hook it up to that old wall wart and leave it on overnight, it will charge if it is not defective. If you really want a good transmitter battery, get a pack of Eneloops, several battery sellers will make up an Eneloop pack for you.