There are two kinds of batteries to use, sealed lead acid (SLA) and Nickel Metal Hydride (NiMH). Sealed lead acid are less expensive and the chargers cost much less. NiMH have a better weight to capacity ratio. NiMH batteries, especially AA cells, have plunged in price recently. There is no point in considering NiCad batteries as they are at price parity with NiMH batteries. NiMH batteries pack more energy in a cell than NiCads, and they do not suffer from the so-called "Memory Effect" (this effect is actually very real but it should be called "voltage depression," see page 40 of The Video Battery Handbook for details). Personally I have no problem with sealed lead-acid batteries for commuting. My 5AH sealed lead-acid battery will run my lights (30 watts), for about 1:40 (measured), which is more than twice the time I need.

Maximum Current Draw
Calculate the current draw of your lamps (Amps=Watts/Volts). Try not to exceed 70% of the batteries AH rating per hour. I.e., on a 5AH battery, try to keep the current draw at or below 3.5A (42 watts of lamps @ 12 volts). For a 10 pack of 2000mAH AA cells (2AH), try to keep the current draw down to 1.4A (16W of lamps @ 12 volts). On my 30W system, using a ten pack of 2000mAH NiMH AA cells did not really work. The current draw was 2.5A, 125% of the AH ratting of the battery. The batteries were unable to supply that much current so the lamps were not at full brightness (and the batteries got very hot). However if I turned on only one of the headlamps the NiMH batteries were fine. To use NiMH cells with 30 watts of lamps I would have to use two packs of AA cells in parallel, or use C or D cells.

Minimum Battery Sizes Based on Total Watts of Lamps

Lamps (W)
Lamps (W)
Total (W) Current
Draw (A)
Minimum 12V
Battery (AH)
Minimum 12V
 Battery (WH)
10 2 12 1 1.5 14
14 2 16 1.3 2 24
15 2 17 1.4 2.1 25
20 2 22 1.8 2.75 33
25 2 27 2.2 3.3 40
28 2 30 2.5 3.75 45
30 2 32 2.7 4.2 50
35 2 37 3.1 4.6 55

Sealed Lead Acid
For 12 volt systems, sealed lead acid batteries are less expensive than NiMH batteries, especially due to the fact that a smart NiMH charger costs $45-85, while a smart sealed-lead acid charger can be purchased for as little as $14. The downside to sealed lead-acid batteries is that they are heavier in terms of weight/Watt-Hour. Sealed lead-acid batteries are rated at 20 hour discharge rates and faster discharge rates (as will be the case with bicycle lights) mean that the actual watt-hours you get will be less than the rating. The sealed lead-acid chargers are cheap enough that you could keep one at work and one at home.

The best selection of sealed lead-acid batteries and chargers, at the best prices are at Keep your sealed lead-acid batteries charged during long periods of non-usage or they will go bad. Do not buy old or used sealed lead-acid batteries. "Removed from equipment" means that the battery is either used or has sat around for a long time. I bought three SLA batteries from American Science and Surplus and all were bad.

Sealed Lead Acid Batteries

2.3AH 3.0AH 5.0AH 7.0AH
7.01" x 1.34" x 2.36" 5.28" x 2.64" x 2.36" 3.54" x 2.76" x 4.02" 5.94" x 2.54" x 3.66"
2.2# 3.0# 4.06# 5.85#
$9.97 $10.37 10.57 $13.87

NiMH (Nickel Metal Hydride)
You can use ten NiMH batteries (AA, C, or D) if you want to spend the extra money. A ten cell AA battery holder is available from Minute-Man Electronics, and is also available from Thomas-Distributing.
Watch out for the thin wires usually used on AA battery holders as they may not be sufficient for high drain batteries. Conventional alkaline cells are not capable of high discharge rates and these battery holders are usually designed for low current drain.

For AA NiMH batteries, do a search for "LENMAR PRO-1020." I have seen these ten packs of 2000mAH AA cells available for less than $10. However the best source, including shipping, appears to be K&B Electronics whose price is $10.79; many of the lower priced retailers have very high shipping charges. has a lower price than K&B, but unless you're ordering several packs then the higher shipping offsets the lower price. If you live near a Fry's Electronics, watch for their sale on Lenmar 2000 mAH batteries, usually one 4-pack for $5. Fry's sells the Lenmar ten pack for $18, not on sale. It used to be that it was better to order NiMH batteries from places like Thomas-Distributing, but now that AA cells have become a commodity item you can find them more cheaply at other outlets. Costco sells a bundle of a Panasonic charger, six Panasonic 2100 AA cells, and two Panasonic AAA cells, for $20, the six AA batteries would be sufficient for a 10W system. Micro-Center sells 4 packs of Sakar iConcepts 2000mAH AA NiMH batteries for $4.94, I don't know the actual capacity of these batteries.

While NiMH batteries can have relatively high discharge rates, it is best to keep the rate at 70% or less than the battery capacity. I.e. a 2000 mAH cell will work longest if the discharge current is under 1.3A. Ten 1.2V 2000mAH batteries will provide 24WH, so try to keep the wattage of the lights less than or equal to 16 watts. Discharge rates of greater than 1.3 amps are not advised as the internal resistance of the batteries is too high for such a high discharge rate. I tried 30W (two 14W headlamps and one 2W tail lamp) on ten NiMH AA cells, and could see that the headlights dim when both headlamps are turned on (plus the batteries get very hot).

NiMH Batteries

click to enlarge: LENMAR PRO-1020 - AA 10-Pack NiMH Rechargeable Batteries

PRO1020 NoMEM PRO C size, button top D size, button top D size, button top
2000 mAH 4500 mAH 7500 mAH 9000 mAH
$8.73     $10.79 $4.50 each 2/$11.99 $7.50 each

Over-Voltaging with NiMH batteries
You can use eleven cells (13.2V) or twelve cells (14.4V) for extra brightness and duration with drastically reduced bulb life. The only charger than can go up to 14.4V is the MH-C777PLUS, but the other Maha charger I listed will do 13.2V packs. The trick of slightly over-voltaging a bulb in order to increase brightness and duration is used by some of the commercial bicycle lighting companies. Use two six-AA battery holders instead of a single ten AA battery holder, if you're going to over-voltage to 14.4V. Use one ten-AA battery holder and one one-AA battery holder if you're going to over-voltage to 13.2V.

The big advantage in over-voltaging is that you get disproportionately more brightness for the increase in power consumption. The big disadvantage in over-voltaging is that bulb life is disproportionately reduced.

You can see a graph of light output versus power versus bulb life at: Over-voltaging by 10% (13.2V) increases power consumption by about 18% and decreases bulb life by about 74%, but it increases light output by about 39%. Over-voltaging by 20% (14.4V) increases power consumption by about 24% and decreases bulb life by about 88%,but it increases light output by about 83%. So a 2000 hour lamp will last about 500 hours if over-voltaged by 10%, about 250 hours if over-voltaged by 20%. Even so, bulbs are cheap, and 250 hours could equal a year of commuting. There are long life (10,000 hour) MR16 lamps, but not in the lower wattages used for bicycle lighting. Carrying an extra bulb is a good idea even when you're not over-voltaging. In short, over-voltaging is a very good idea as long as you are willing to accept the consequence of more frequent bulb changes.

Parallel NiMH Battery Packs for Longer Duration
For longer duration, and/or higher current capacity, use higher mAH batteries, or use multiple battery packs. While higher mAH cells (i.e. C cells or D cells) are technically a better solution, the advantage of using AA cells is that they are very inexpensive (C & D cells have not come down in price nearly as much). One good thing about NiMH battery packs is that you can connect them in parallel (same voltage). You can connect two 12-14.4 volt packs together and double your duration. Each pack must have the same number of cells and you should use the same type of cells (same capacity) from the same manufacturer in each pack. Both packs should be fully charged when connecting them together. You do not need any isolation between the packs. There is one caveat: 


Is that clear? Separate the two packs and charge each pack on its own.

Serial Battery Packs for Longer Duration
You can always carry extra battery packs and simply install a fresh pack when the first pack is exhausted.

NiMH Battery mAH Ratings
Every brand of NiMH batteries is different. A 2000mAH cell will have real mAH rating from 1754mAH to 1856mAH (according to an independent test). You can see ratings of various NiMH AA cells at: Perform a cost/mAH analysis on the various batteries. For example, the Lenmar 1754mAH AA cells cost about $0.90 each, while the Powerex 1856mAH AA cells cost $3.50 each; nearly 4X the price for less than a 6% increase in capacity (and in the duration test there was less than a 3% difference).

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Battery Case

Wide-Mouth Water Bottle
This method is used by many of the commercial lighting system manufacturers; they like it because it is very inexpensive, does not require clamping or strapping the battery to the frame, and is easily removable, which many commuters require. I hate it because I want my water bottle cage free for a water bottle. You may need to formulate the battery pack to fit the bottle, which may mean buying batteries with solder tabs and arranging them in a cylinder pattern (this is more expensive than buying standard AA cells, but not significantly more). The wires come out through the nipple. You could also put the switches through the side of the bottle. Of course you lose the use of the water bottle cage for a real water bottle or coffee bottle. Not to worry, there are many ways of adding another bottle cage, see my web site Adding Water Bottle Cages to Bicycles without Braze-Ons for details.

A water bottle (or coffee bottle) belongs in your water bottle holder, not a jacket, bottle of air, cell phone, tools, lock, or batteries.
It is quite annoying how so many accessory manufacturers think that the water bottle cage is fair game as way for them to get around providing their own mounting hardware. I don't want to give up my water bottle cage for a battery, cell phone, tools, compressed air, jacket, etc. Your mileage may vary, but what are you going to do when you're thirsty or hot, drink batteries or tools? While in the past, road bikes and touring bikes usually came with two or three sets of braze-ons for cages, this is normally not the case any more, in fact on some compact frame "women's" bicycles, the single set of water bottle cage braze-ons isn't even usable without a special rotating bottle cage (see below); my wife bought a Specialized Expedition and got the "man's" model, solely because of the water bottle issue, but few people would even have realized the problem at the time of purchase, and even fewer would have been aware of the workaround.

Many different accessories compete for your water bottle cage
(Click Images for Details)

Cell Safe DX4 Phone & GPS Case tire/tool pouch for bottle cage
Jacket Bag

Use Profile Cage for best fit

Airzound II

Use Profile Cage for best fit

Cell Safe DX4

Use Profile Cage for best fit

Tool Bag

Use Profile Cage for best fit

Cygo Lite Night Rover 12 watt 6 volt Gel-Cel Lightset

29pc Water Bottle Bicycle Tool Set  Master Lock Street Cuff Bracket Fuel Bottles (for bicycle camping)

Use Profile Cage for best fit

This cyclist, cooling off on a Sonoma County trail, is glad to have a water bottle rather than a bottle of tools or batteries! Unfortunately, this is not a common sight!

The profile cage is preferred for many water bottle cage-mount accessories because it grips the bottom of the bottle only. Many water bottle type accessories don't have the indentation for the angled section of a metal cage to go into.

Click Image for Details

Profile Cage, $8 at Bike Nashbar

A solution for bicycle frames where a normal water bottle cage is not usable
Click Images for Details

This "women's" frame bicycle has a useless set of water bottle braze-ons unless you buy one of the new rotating bottle cages (the braze-ons are right above the "S" and the "E" in "Specialized" on the lower down tube). This unique bottle cage solves the problem of being able to use a water bottle on some "women's" bicycle frames. $16. This cage from Specialized solves the problem of being able to use a water bottle on smaller frame bicycles. $10.

Frame Bag, Seat Bag, Rack-Top Bag
The battery or battery pack is placed inside a frame bag or seat bag. Use a rack-top bag that attaches securely with buckle type fasteners, not Velcro. Not only because the battery is heavy, but because the Velcro type of attachment doesn't retain its strength over years or removal and attachment.

The Lone Peak RP-700 attaches with buckles, not Velcro. I went through two poor quality rack packs before spending the big bucks on a higher quality product with superior design features. I'll let you know in five years how it holds up!

Alas, my opinion of Lone Peak has been tempered by a recent experience. One of the straps on my wife's RP-700 fell off, and I sent the pack back to Lone Peak, expecting it to be fixed for free under their lifetime warranty. Instead, they claimed that the pack had been abused, and charged me for the repair, and the return shipping. It wasn't a lot of money, but it was petty of them to charge me, and it was annoying for them to claim that somehow the strap falling off was my fault.

Plastic Case
The battery or battery pack is placed inside a plastic case which is attached to the frame with some sort of clamp, i.e. the Zefal Gizmo Clamps, padded U bolts, etc. This is more permanent mount which is less likely to be stolen if left on the bike. You could put the switches through the side of the box as well, eliminating the handlebar controls. This approach makes battery changes more difficult as you'll have to remove the lid of the case, so it's not a good approach if the duration of your commute requires battery swaps (though you could put multiple battery packs inside and have SPDT switch to select between the two (while it is fine to connect two battery packs in parallel for running the lights (no isolation is needed), it is NOT fine to charge two packs in parallel, so a switch is necessary)). It also requires that the charger be located close to the bicycle.

Inside Seat Tube
I've never tried this, but AA cells will fit inside a seat tube. You'll need to buy solder-tab type batteries, make a ten pack, seal the pack inside heat shrink tubing with wires coming out through the seat post. You may want to use some foam to prevent rattling. If the top of the seat post is sealed then you'll need to drill a hole for the wires.

 Images for Details

Frame Bag Seat Bag Wide Mouth Water Bottle Plastic Enclosure
6"x4"x2" 270-1806
Big enough for two AA battery packs (smaller enclosures available)
$3 $6 $5 $5

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