My family goes car based camping a few times a year. I also go pack carry camping as well, but that requires a different set of gear. A number of years ago I purchased a 7W fluorescent camping lantern. It has worked well, but now the battery is cooked. Here’s a look at the box of the product.
Designed to fail
The cheaper – and not so cheap – lanterns can often have a poor design that can lead to early failure because they do not treat the internal battery appropriately. These units are often supplied with an internal sealed lead acid battery (SLA). My one had a 4Ah 6V battery. This can power the lamp for about 3 1/2 hours. One problem is that SLA batteries should not be cycled more than 50%. They also should be stored fully charged. The 50% cycle means that this lantern should have only been used for 1 3/4 hours on a full charge, and then it needs recharging. No mention of that in the small manual that came with the lantern.
The second problem is that the charging circuit built into the device does not charge the Pb battery in a way that is according to spec. The charging voltage is too high. It’s going to cook the battery a little bit on each charge.
So now I have a lantern that does not work. Should I just throw it away?
The rest of the lantern seemed ok, it just didn’t have a working battery. I could have just bought a new battery from the supplier, but they charge more than the cost of a new lantern. So, I thought I would replace the battery with something that was more suitable.
Firstly, I have been using LiPo packs in Amateur Radio applications – powering a low and a moderate power radio. A 3S – 3 cells in series pack has a voltage of 10 to 12.6V. Too high for this lantern directly, but it could be converted to 6V. I found some LiPos at Hobbyking that would do the trick.
Three of these fit into the battery area of the lantern quite snugly. I cut the main cable to a short run and used JST-XH connectors, which are shown in the picture above, as these connectors are the same on my other LiPo’s – makes it easy to charge on the chargers that I already have.
Having three of these inside, with 6VDC regulation, gives me capacity over 3 times the old battery, but I can use nearly the whole cycle, so it is really 6 times the capacity.
I grabbed a DC converter off Ebay that outputs 6V up to 3A. The lamp draws about 1.2A, so that is easy going for this converter.
I would not be using the existing charging circuit. It was no good for the Pb battery, and it certainly would be no good for these LiPo packs. I could make use of the LED in the lantern that indicates when the lantern is charging. Instead, I would use this LED to indicate that the voltage in the LiPos was good. The 6V converter would work with an input voltage of 7V, which would mean the LiPo packs driven down to an average of 2.3V per cell – far too low. Instead, I would use this LED to indicate when the packs were getting flat, at around 11V or just a little under. The LED would be on when the battery was ok, and go off when not.
I needed a simple circuit to drive the LED. From the LiPo voltage source, I fed a 470ohm resistor and then three Zeners. I selected three, because the breakdown voltage of around 11V was what I needed, and I had these on hand. Three of these zener diodes, each with a breakdown voltage of 3.6V, in series gave me what I needed. The current on the zeners would be about 3mA – set by selecting the 470 ohm resistor. Connected between the Zeners and the resistor is the base of a 2n4401 transistor, with the LiPo voltage on it’s collector. From the emitter of the transistor, I had a 1K resistor. This would have typically about 9V across it, meaning a current of about 9mA. This was in series with the LED, which has it’s own 2V drop. So from the LiPo voltage, the transistor would have a drop of around 2V when the LiPo pack was fully charged, falling to zero when flat, plus just under 9V for the resistor and 2V for the LED.
This circuit was put onto some veriboard and put into the lantern’s battery compartment. It is shown below and the DC converter is immediately beside it. Below is the factory built charging circuit that is now unused.
Unfortunately, I mixed up one of the JST-XH connectors on one of the LiPo packs, with the plastic housing having wrong polarity. I made one of the associated connectors to match it, and marked them both with tape so I don’t use it for the other packs. I’ll just have to live with it.
After all of that, it was time to turn it on. Works well – very pleased with the outcome. Now, I can safely use the lantern for about 11 to 12 hours between charges, rather than under 2 hours. This has turned into an upgrade rather than a repair of the lantern.