Building a basic Lipo charger?
 

Hey guys, i'm looking into making rechargeable transponders for my local RC track. Right now i've taken there AMB transponders and wired them to a 1s 100mah LiPo witch should last 5-6.6 hours on a charge, plenty for the normal 3 hour race day. The problem is charging them. I am a machanical engineering student that's good with circuits (relatively speaking) but I don't have the in dept knowledge for the logic circuits.

The battery is rated for 2c charge rate, of witch i would rather only do a 1C charge to ensure better battery life. All the 1s chargers i'm finding online are made to charge faster than 100mah, witch I don't want.

Questions:
1: Can I simply make a voltage divider circuit so the battery sees a 4.2V source (at steady state) all day long without damage to the battery life? It's not really a trickle charge and LiPos are generally known to no develope a charger dependence, but i'm not sure about this one. If so, problem solved just by using 2 ~95ohm and 1 100ohm resister for each battery and they can't exceed a .1A charge, but they will take forever to charge towards the end.

2: If I use a premade charger with a simple resistor to slow down the current, what would happen? If increasing the resistance the charger sees around the battery only slows down the process, problem solved. The charger will always see a higher voltage than what the battery is actually at but i'm not sure if the charger will slow down at the end of the charge, allowing it to still fully charge or if it'll just go full speed until the voltage reading is 4.2 then stop, leaving the battery undercharged.

3: Using components (IC's, capacitors, ect.) already made and assembling a basic circuit, does anyone know an effective and cheap way to build a 1s LiPo charger from scratch?

For reference, here are the closest pre-made charger I can find so far.

.15A (1.5C) - workable, but would still prefer a lower current and price for each one.
http://www.ebay.com/itm/301326728381...%3AMEBIDX%3AIT

I would go for a 5v power supply like a high power USB charger or a 5v-5A cheap PSU from ebay and some lithium charger boards:

http://www.ebay.com/itm/10Pcs-Lot-TP...ht_3689wt_1125

http://www.ebay.com/itm/5V-Mini-USB-...ht_6344wt_1145

These chargerboards have a full automated single chip lithium charger. If you know the chip you can find the specsheet to help you to change the charging current when needed. (mostly a resistor)

I wondered about those, but need to look into how to adjust them

http://www.tpmicro.com/goods.php?id=47

Wait, i though changing the resistor changed the current?

http://lygte-info.dk/review/Review%2...4056%20UK.html

Yes. Rprog in the diagram. ore info:
http://lygte-info.dk/review/Review%2...4056%20UK.html

Just change the resistor to the right value

ok, i don't see the label for rprog, but it's circled in the picture so i know witch on it is.

Thanks for the information, I ordered 10 of them to start with. Just got to wait a while