Light Battery homebrew
 

The battery for my Planet Bike Insight 15W light is toast - doesn't hold a charge much and run time is down to 1/2 an hour. The battery is a 6V 4.5 A/H NiMH. The replace battery is $100 and I have no where near that kind of bread.

Can I use 5 "AA" rechargeable batteries? The ones I have here are NiMH 1.2 V 2650mah. Can I use five of these and if so, how would I attach them all together? And then attach them to the wire that hooks up to the light and to the power/setting button?

Thanks a lot! Winter is coming!!

Better to use a 4.5 A/H sealed lead acid battery, probably $15-$20 & it will last you through the Winter.
(edit: Batteries.com shows 4.5 A/H for $16.50 @ 1.75 pounds weight & a 7 A/H for under $20 @ 2.8 pounds weight. You should be able to find similar locally & avoid the shipping cost)

On the other hand, Ronocerous, you could get a 3.8, 4.2 or 5.0 ahr NiMH from BatterySpace for from $25 to $30. The battery is lighter, smaller, more rugged, and can be deeper cycled then a sealed acid. A much better choice.

You could also go to All Battery and get a 7.2V RC car battery with a 4.2 Ahr rating for around $30. They also have a 6V 5.0 Ahr NiCd battery for about $30 which is even more rugged than the NiMH. If you go with the higher voltage battery, your light output will be better at the cost of a slightly shorter bulb life. Output is about 50% higher.

Thanks for the ideas! So, does matching the 4.5 A/H matter that much or is there some flexibility there? R

Yes, you can
 

Wire them in series (connect + to - in a chain, connect the end - and + to the charger/light). You can connect to banks of these in parallel to extend the burn time. (connect + to + and - to - of the two chains)

If that makes no sense let me know I'll draw up a diagram.

The 4.5 A/H tells you how much the battery stores... if the voltage rating is the same, but with a larger A/H rating, you'll get the same amount of light but with a longer run time.

So... no,, you DON'T have to match the amp hour(A/R) rating... the voltage rating of the new battery should be close to the old one, however. Slightly higher voltage will give a brighter light, but burn out your bulbs sooner.

For an MR11 halogen the life is shortened but it's not really that significant. I've been doing this for years to these lights and this year was the first time I've ever blown a bulb. I figure I got about 8 years of use out of that bulb. I might have shortened the life by a couple of years...no big deal really.

One thing you can do, Ronocerous, is to by batteries with a lower Ahr rating and either wire them in parallel or, perhaps better, switch them out when one starts to dim. I've done that for ever too. At All Battery, the low Ahr rating batteries are slightly cheaper.

To get an estimate of the burn time for a battery, use the following equation:

Hr run time = (Ahr x Volts)/watts of bulb

This is the best you can get but in real life the time is probably a bit shorter. New batteries, in particular, take few cycles to get to full charge.

Considering that you have problems with the charge on the battery, you might want to look at a new charger too. For NiMH, you should have one that monitors not only the charge but also the temperature of the battery pack.