Using 18650 Lithium batteries in the transmitter

[JLPicard]

Would it be possible to perhaps eliminate the AA batteries in the transmitter and utilize a couple of 18650's in parallel to power the unit? I would then like to build a solar charger for the 18650s. Here is a link to a place that sells them: https://www.dealextreme.com/p/trustfire ... pack-19770

Since they are rather much larger than the AA, I would probably have to build a weather proof container to house them. They are 3.7 V. Do you think replacing a 3.0V power supply with a 3.7v supply would effect anything?

Sorry I think this post should be moved to Homebuilt please.

[AllyCat]

Hi,

This may be the correct thread if you're asking if a FO unit is likely to tolerate a 3.7V nominal supply voltage?

Firstly, rechargeable Lithium batteries in AA size are available, so you could fit 1 or 2 in the FO box. The ones I have are numbered LC 14500 and rated at 900mAh, 3.6 volts. I'm not sure how much (average) current the USA (915MHz) versions consume but my measurements (on a 434MHz unit) suggest a single 900mAh cell should be ample if combined with solar charging.

Many people use a pair of (non-rechargeable) 1.5 volt Lithium AA cells which are excellent at low temperatures and should run the FO unit for at least 3 years. These cells actually start at 1.85 volts, so the FO unit is presumably "safe" to 3.7 volts. BUT "3.7 volt" lithium rechargeables actually charge up to 4.2 - 4.3 volts (that is the required constant voltage charging specification) !

I do not know if the FO unit is "safe" at that voltage. Some of the circuitry is probably the same as in the base station (LCD) which uses 4.5 volts (nominal), but I can't guarantee that there isn't a "critical" component somewhere in there. A possible solution would be to replace the second battery with a dummy circuit which drops about 0.7 volts. A forward conducting diode might be suitable but you need to bear in mind that for about 44 seconds (of each 48 second transmit cycle) the transmitter consumes almost NO current so a simple diode "dropper" circuit might allow the voltage (on any internal supply decoupling capacitors) to creep up to above 4 volts.

Cheers, Alan.

[JLPicard]

Hi,

Firstly, rechargeable Lithium batteries in AA size are available, so you could fit 1 or 2 in the FO box. The ones I have are numbered LC 14500 and rated at 900mAh, 3.6 volts. I'm not sure how much (average) current the USA (915MHz) versions consume but my measurements (on a 434MHz unit) suggest a single 900mAh cell should be ample if combined with solar charging.

Cheers, Alan.

Thanks for the quick response Alan. What sort of length of time do you get from your 14500's before they require recharged?

[AllyCat]

Hi,

I'm not actually using the Lithium AAs in my FO station. However, I have measured the power consumption of my (434MHz) transmitter, which is about 600mAh per year. "American" (915 MHz) and "European" (868 MHz) transmitters may consume rather more power, but I'm sure a single 900 mAh cell should provide sufficient energy over even a very dark winter (between summer recharges from a PV solar cell).

The issue of battery life is quite complex. Alkaline (and other cheaper) cells start at about 1.6 volts but the voltage falls steadily during life and the manufacturers don't consider them exhausted until they reach typically 0.9 volts. NiMH and NiCd (rechargeable) cells start at about 1.3 volts but remain above 1.2 volts until at least 80% exhausted. So the "life" obtained in the FO station depends on how much voltage the transmitter requires (e.g. to achieve the required range). My transmitter sends a "low battery" flag (note that the base station does not necessarily report this) at about 2.5 volts (i.e. 1.25v per cell) but seems to work fine down to about 1.9 volts.

Some people have reported a battery life of only a few weeks/months, but to my knowledge have never reported whether this proved to be due to a faulty battery or transmitter, or "dirty" battery contacts, etc..

I had considered posting my investigations into the design of a very low cost "solar power upgrade" but all rechargeable cells have limitations (voltage, leakage, temperature, etc.). It's likely that a pair of (non-rechargeable) Lithium AA cells (costing just a few $/£s) will last for up to 5 years and work excellently at very low temperatures. By which time the battery contacts will probably have corroded, or some other part of the FO hardware failed in some way.

Cheers, Alan.

[The QCC]

Hello JLPicard.

I was using two 3.2V, 400ma rechargeable Lithium batteries in parallel in the FO transmitter. A very easy mod to the battery case contacts. The 3.2V cells are AA size.

I used the solar panel and charging circuit from a Westinghouse outdoor solar lamp that uses 3.2v Lithium cells to charge the batteries.

Note the past tense. I reverted to standard Lithium AA cells for the transmitter.
Now I am using the solar panel and charging circuit to power a fan in my homebuilt solar shield.

The trade off is in favour of a ventillated shield. Lithium batteries have a relatively long life.

Carl

[JLPicard]

Carl, were you using 14650's? If so they are 3.6 or 3.7 (reported at both voltages by vendors. I think I will go that route and rewire the contacts in the transmitter for parallel operation. I have a pretty good stack of 'slightly' damaged solar cells, I am planning on soldering them together until I get the prescribed voltage to charge the two 14500's. Anyway, that's the plan atm.

[The QCC]

The batteries I used are Lithium Phosphate Model 14500, 3.2V 400ma. They are AA size and fit the FO battery case. Care must be taken when using these batteries as they fit any AA size battery case.

My original installation used the solar panel from the Westinghouse solar light.

The solar panel I am using for the aspirating fan is from a solar powered water fountain, 7V @140ma. It is weather proof, comes with mounting points on the back. From Ebay costing $12.70 delivered. The fan I use is one of the dual fans in a "USB laptop cooler". From Ebay costing $2.55 delivered.

Buying a solar pump with a weather proof panel eliminates a lot of tinkering to make thing weather tight.

Carl

[Charlie]

A basic consideration is what your objective is in adding a solar charger.

If it is making the time between having to service the gear in some fashion as long as possible, I suspect you are actually making things worse. The F.O. transmitter batteries basically die from shelf life as it uses so little power (almost). Depending on the distance between transmitter and console, standard alkaline batteries should get you 1-2 years, 1.5V lithiums should get you 5-10 years. In truth, the F.O. transmitter likely will die before that - it's chip on board technology isn't exactly noted for long life. Add to that the fact that your mods add more connections to potentially fail, new corrosion points, and simply more parts, for me at least when I considered this 2 years ago, the mod made no engineering sense at all.

My history... put in alkalines to start. At about 9 months, I switched to lithium because I was moving the unit to a more inaccessible location. The alkalines went into something else as they were not noticably discharged. The lithiums have been in use for over a year now. During the time I've owned the station, I've had 2 transmitter failures - the first at 14 months, the second 6 weeks after that, then things have been fine again. YMMV.

A solar charger falls into the category of "seems like a good idea", that in practical terms turns out to be of little value.

[kodi]

I had a transmitter, where battery were drained in about 10-12days. It occured that transmitter was just faulty, but in the meantime I built pretty powerful solar charger (up to 1.5W) with voltage regulator from Maxim. Li-Ions were fine to use, as the voltage on the regulator was set to 3.6V. But - in the meantime it started to consume more and more power, so even 7Ah battery pack was enough for about a month. Then I decided to replace my transmitter with one that has included solar cell. Transmitter arrived with included AA RAM (Rechargeable Alkaline Manganese) cells. They standard voltage is about 1.45V and can be trickle charged almost infinitable (but be warned - deep recharge cycle can be repeated only 10-20 times). I installed the thing and since then - forgot about transmitter maintenance.