brundle_fly

Would this be correct:

If I wire two 7.2v 4Ah Nimh batteries together(+'s and -'s) in series? I'd get 14.4v 8ah
Also,will I need a 7.2 charger or 14.4

sherpa93

Simple battery wiring Answer
Series wiring + to - will double voltage

Parallel wiring - to - and + to + double amps

Cant have both sorry.

Battery charging works on impedance. For instance to charge a
12 volt battery you need at least 12.1v usually slightly above
battery voltage (14-18v as in autos).

Other than that battery chargers are designed to kick out certain amps
to not overcharge/overheat a pack rated at given mAh.

More to it but thats the short skinny version.
Good luck on your project

brundle_fly

Thanks a lot for that.That diagram clears it all up.I suppose 14.4v 8ah batteries are pretty expensive if it's possible to get one that is?I may just stick with a 7ah SLA if it's going to be the most cost effective way

fordfasterr

What kind of light will you be using this 8ah pack with ?

brundle_fly

I plan to use a 20w,or maybe up to 35w Mr-16 Masterline depending on brightness.
I hooked a 35w up to my car battery and it seemed more than ample.
Do you use 24 or 12 degree beam for road purposes.This one I've got is 36 and throws a pretty wide beam

fordfasterr

I currently have the stock (20 watt) bulb that comes in the garden lamp from home depot ($ 9.99) (malibu?)

I am using a small 5Ah LAB

I would like to get the 12 degree beam but I don't know the part #.

Originally looks like this:

moxfyre

You will get a 14.4V battery with a capacity of 4Ah. The capacity does not increase because when you put the batteries in series, you are combining them to produce a larger EMF (electromotive force), rather than to produce a prolong their life. If you put them in PARALLEL, the voltage will be 7.2V, but the two batteries will "share" the task of generating current, so they will be able to produce a given current for twice as long. (You can read up on Kirchoff's Circuit Laws for the details.)

You can charge them with a 7.2V charger in parallel, or a 14.4V charger in series. If you don't want to remove them from the series circuit in order to charge them, then go with the 14.4V charger.

cyccommute 

One thing to consider, however is that with the lower current draw, a 20W bulb on a 14.4V system and a 4 Ah battery should give you 2.9 hours of burn time, ideally. Two batteries running a 7.2V system in parallel will give you the same burn time. If you need an 8 Ah battery, you could run 2 each of the 7.2 in series and then 2 of those packs in parallel.

I've done what you want to do brundle-fly. Here is the 14.4V system and wiring



The negative of one pack is connected via a jumper to the positive of the other pack.

If you want to run them in parallel you can do this



For a charger, I use a Maha C777 Plus. It's not cheap but it automatically adjust the voltage to charge the system. I can charge a 14.4V system on it, unplug the battery and charge a 7.2 V system without doing anything to the charger.

Finally, for the bulbs, a 24 degree bulb is probably the widest you want to go. A 24 degree bulb will give you a pool of light that does well for illumination of the ground directly in front of your wheel. A 12 degree spot will put a beam that highlights stuff further out. The beam pattern on the 24 is probably 10 feet wide at 10' in front of you wheel. The 12 degree has a beam pattern of around 4 feet at 10' in front of your bike.

moxfyre

Indeed! Two 7.2V batteries in series will of course have twice the ENERGY capacity of a single 7.2V bulb. But the current capacity will be the same. It's rather misleading to have the capacity of batteries labeled in "A.h" rather than, say, "Joules"... since of course a 9V/500mAh battery holds more ENERGY than a 1.5V/2500mAh battery. But I suppose it is convenient, since most devices need a specific supply voltage, and so you can simply compare the current capacities of different batteries.

Also don't forget that a "20W" bulb may not actually draw the same power at 14.4V as at 7.2V! That's to say, if it draws 1A at 7.2V, it doesn't necessarily draw 0.5A at 14.4V. The efficiency of the bulb may vary with the supply voltage as well... depends a lot on the particular light technology.

ken cummings

I use a 10 degree beam angle from a 50 watt MR16 bulb. Produced by Sylvania The Malibu type garden lights I have seen in several local stores are mostly plastic and need to be fair to middling large so they have enough surface area to shed waste heat. Plastic holders are ok for weaker bulbs.