Building a bottle battery

If you read my blog, you’ll have seen my recent review of my RSP Asteri 2 bike lights. I talked about making a bottle battery, and now I have.

Unfortunately I forgot to take photos after the first couple of steps but never mind.

What you’ll need

I used the following parts, but you can use whatever you like.

A note on batteries

• It doesn’t matter what kind of rechargeable batteries you use – NiCd, NiMH and Li-Ion are all fine.
• It doesn’t matter what size of batteries you use. If you wanted to make a smaller “pouch” of batteries you could use C type.
• You must have precisely 4 batteries in each series loop, but there’s nothing to stop you using 8 batteries and having two series loops in parallel.
• The capacity (measured in mAh) is an indicator of how long the batteries might last.
  • The batteries that come with the RSP Asteri 2 are AA NiMH batteries; 1600 mAh each. The manufacturer reckons you can get 1-2 hours on full power and 6 hours on flashing.
  • Inexpensive D type batteries might give you around 3000 mAh, which immediately doubles your battery life.
  • My D type batteries give a claimed 8000mAh. Scaling up the manufacturer’s estimate, I might be able to get 5-10 hours on full power, or 30 hours flashing. This basically gives me the ability to do a week’s commuting on full power without recharging until the weekend.

Building it

I cable-tied the battery holders back-to-back as they happened to have holes in the right place. A pair of D batteries just about fitted through the neck of the bottle if I squeezed the bottle. I attached the batteries in two pairs so they could sit on top of each other.

I soldered the battery holders together in series using the single-core wire. I used fairly long stretches of wire (about 8″) between the upper and lower pairs of batteries, to make it easier to feed the batteries into the bottle.

I taped up the exposed solder with insulating tape so there was no chance it could make a short circuit if the batteries moved around once in the bottle. I packed the batteries into place with bubble wrap so they wouldn’t rattle and risk damaging the solder.

At the neck of the bottle, I terminated the two single cored with a terminal block and connected the twin-core cable to the other side. I threaded it through the spout of the bottle and screwed the cap on. The terminal block also allows you to easily reverse the polarity of the circuit if you accidentally wire it back to front for your LEDs. 😉

I measured the trailing cable to make sure I had the right length, before trimming it and soldering the DC power connector on the end.

The screw cap seemed tight and waterproof so I left it alone, but I plugged the gap around the cable through the spout with hot-melt glue gun.

The batteries I bought claim to come fully charged so I was able to test it immediately. Which brings us on to our next section.

Recharging

Obviously it’s going to be a bit of a pain to remove the batteries to recharge them, so we will charge them through the neck cable. Buy a female DC socket to match the plug you bought for the bottle battery. Also buy a female connector to match the RSP charger’s plug. Solder them together and ta-da! You have an adapter cable to connect your battery to the charger. Watch the polarity here, or you might set fire to your house.

If you don’t want to take the risk of breaking your original RSP charger, any DC power supply capable of supplying 7.5V with at least 300mA should do the trick.

Considerations

The original RSP battery had a warning LED for low battery. You don’t, so be careful you don’t over-discharge the bottle battery or you might damage the NiMH cells. As soon as the LEDs start to go dim, switch them off. You would, of course, be wise to keep a regular battery light in your bag in case of emergencies. I keep a small Cat Eye HL-EL510 for situations like these.

The original RSP battery had automatic charging shutoff when it was full. You don’t, so be careful not to overcharge the batteries. Probably 12-18 hours will be enough. Check the instructions that came with your batteries.

You can get around both of these considerations if you pilfer or copy the PCB in the top of the original RSP battery pack. Again, depends if you are willing to sacrifice your original battery pack, and if you can be bothered with the effort.