Lipo bible

[Crazy L]

Hi guys

I was considering upgrading my leccy gear as My RC2000s are 8 years old now and my chargers are to suit. I was trying to swot up on Lipo but no one post answers all my questions like: whats the config of stick/saddle packs? (2s2p) etc and what it all means. C ratings?, good chargers for little £? Basically, a kind of beginners bible for Lipo. I'm looking at 1/10th off road usage btw, in 4wd. So a little advice maybe, but please, no "I only use blah cells/charger cos they are the best" crap.

So please, if you have some time to spare, a numpties guide to Lipo would be cool as I'm a little stuck in the mid '90s at the mo:D.

[glypo]

Stick packs are 2S1P and are the most common form of LiPo pack in the whole industry, and as such offer best value for money. 1/10th off-road is really the only class that uses saddle packs, and unfortunately they offer worse value for money. However if you have a saddle pack car obviously a saddle pack battery is a necessity and not a choice. Saddle packs are either 2S1P or 2S2P depending on the manufacturer if I recall (where they are actually two individual 1S1P or 1S2P joined together). The actual battery configuration, i.e. 1P or 2P makes no difference however as they will always be 2S (7.4V) for racing, so it’s the C value that should concern you.

The C value means the same for LiPo as it does for NiCad and NiMH so not sure why people find it confusing? Unless of course people don’t pay attention to C values on their normal cells. Anyway the C value relates to the capacity of a battery in terms of charge/discharge rate. If a battery is 4900mAh (i.e. 4.9Ah) then 1C means 4.9 amps. So if your battery says charge at 1C, and it’s 3600mAh, it means charge at 3.6 amps. Very simple really, isn’t it?

The importance of C values come with discharge though. Say your battery has a 15C rating, and is a 3600mAh – you do the basic maths. 3.6 amps x 15 = 54 amps. This means the battery can deliver 54 amps. If you have a motor/esc that can draw 65 amps though, then this battery is not good enough and you will either have a slow car, ruin the battery or both. The higher the C rating on discharge, almost always means the lower the voltage drop under load as well, which gives better performance.

Typically there are at least two different C ratings. One for continuous use, which is the lowest C rating, and then one for acceleration, known as the burst rating. This rating is higher. For example the TrakPower 4900mAh has a 25C average rating (122.5 amps) and a 30C burst rating (147 amps). This means it can handle 122 amps average, but if you have a very thirsty esc/motor it will be able to supply up to 147 amps for a short period (3-4 seconds usually) for heavy acceleration.

A final note on C ratings. LiPo battery packs do not advantage from being cycled, and should never ever be peaked after they are charged (they do not lose charge like NiMH etc). However, LiPo battery packs do deteriorate after use. You will see someone like TrackPower say up to 100 race-worthy cycles. This doesn’t mean the cells will die after 100 cycles, it means they will become less effective, so effective C rating will drop. So if you buy good C rating cells (above what you need), when they start to deteriorate you will get longer life out of them. Also note, 100 cycles is LOADS, a lot more than it sounds. As you don’t cycle LiPo, and say you use 1000mAh per race and you top-up after each race, you might only use 6000mAh for the whole race meeting, which for a 4900mAh pack would be classed as 1 and a bit cycles, so you can get nearly a hundred race meetings (nearly 2 years if you race once a week) so it’s not as bad as it sounds!

It’s worth getting a good charger for LiPo. LiPo is almost totally safe under discharge really, it’s the charge process that is the most critical in terms of safety. If you buy a cheap charger, and even a lot of the racing chargers are just rebranded cheap Chinese knock-offs, you risk damaging your cells or worse.

A balancing charger is well worth it, these will balance the two cells in the pack on charge, and ensure they stay in top condition. Racing can draw high currents, and it is this high load that can upset cell balance. Something like the Bantam eSTATION BC6 will balance the cells on charge as quickly as a normal charge – so with this you can balance your cells on every charge which might help the cells last that little bit longer. My Review (http://www.glypo.com/reviews/bc6/) - Price. (http://www.m-k-racing.com/product_info.php?cPath=54_102&products_id=5474)

Of course there are plenty of charging options, I just recommend you get a good quality one - and one that can balance charge. And beware of some chargers from some racing companies which are just re-branded poor quality.

Cell quality is dead important too. Cheaper cells often claim C ratings which are totally unrealistic of what they actually achieve. The best BRCA legal cells are most likely TrakPower. TrakPower/FlightPower use Enerland cells inside, and assemble to the best standards for low internal resistance. FlightPower has exclusive rights to Enerland cells in the UK, and as such you won't find them in other packs which are rebranded.

The other options on the BRCA approval list are limited. Orion use Kokam cells, which are fine. Kokam used to be a big player in LiPo cells (talking from my rc plane experience here, where we have been using LiPo much longer than the car market) but in recent years Kokam have not really matched the performance of others, and as such the specs on the Orion packs aren't as impressive, and the cost is higher too.

If you have stick fitting in your car, then the Demon pack is becoming increasing popular. It's BRCA legal, and £50 for a 5000mAh which is good going. I think it has a JST-XH balancing lead on it too, which makes it compatible with a great number of balancing chargers. I have no idea what cells are in the pack though. I would still rather spend £15 more on the better spec TrakPower though, not that the Demon pack isn't great value and works just fine, just my preference.

HTH.