More about connecting packs in series and in parallel
It's probably worth pointing out that although adding battery packs in parallel adds the capacities together, connecting them in series doesn't - it just adds the voltages together. Two 1000mAh 2 cell lipos in series is just a 1000mAh 4 cell lipo. Two of them in parallel, is a 2000mAh 2 cell lipo, and so on.
When connecting packs together in series or in paralled, the C rating doesn't change - at least if they're both the same C rating. If you connect two 1000mAh 10C 2 cell lipos in parallel you get a 2000mAh 10C 2 cell lipo - the amount of current you can draw is doubled, because there's twice the capacity, but it's still 10C, not 20C.
Much more importantly:
Looking after lipos
- don't over discharge
People who are used to NiCd and NiMH packs are used to squeezing the last bit of useful power out of them before they recharge them. For NiCds that's definitely the best thing to do, and for NiMH packs it's good to at least do it once in a while, and doesn't hurt to do it every time.
If you do the same thing with lipos you will kill them very quickly. Lipos hate being over discharged. You may not notice the difference straight away, and they'll still charge up, but the capacity of the pack goes down every time you do it, and the C rating starts to fall too.
- keeping them healthy for longer
So, if you want your lipos to last, it's far better to just use 40%-50% of the capacity at a time, then land and put in a fresh pack. Relying on the low voltage cut out in your ESC is generally not a good idea with lipos, for that reason. Arguably the best way is to check your lipos after each flight to read the voltage, and time your initial flights - start with a few minutes, check what's left in the lipo and decide whether you can stretch it a bit more next time. Allow for windier weather by reducing the length of the flights. A plane that can happily fly for 15 minutes on a calm day without wearing out the battery pack might only manage 5 minutes in a stiff breeze.
After a while, you get to know how long you can fly a particular plane. Personally, I think it's still good practice to check the pack voltage after every flight so you don't get complacent, and so you notice when a pack is starting to wear out.
I aim for no less than 3.8V per cell after a flight. A lipo checker that plugs onto the balance lead to read the cell voltages is very cheap to buy and will save you a lot of money in knackered lipos. The voltage you measure after a flight is a lot higher than when the motor is running. It's important not to let lipos fall below about 3V per cell under load. That will look more like 3.7V per cell when you measure it afterwards.
- charging more slowly is better, and safer
If you keep charging a worn out lipo assuming it's the same capacity and C rating as when it was new, you may well have a fire. It's safe to charge any lipo at 1C (e.g. 1000mA for a 1000mAh lipo) but as the capacity and C rating falls with wear and tear, the current you used to charge it at becomes too much and it starts to get hot when you charge it. If it gets too hot, it'll either explode or do a good imitation of a marine flare. It won't burn for very long, but it will burn very hot and you won't be able to put it out. All you can do is stop the fire spreading to things around it til it's finished.
While charging at 1C is safe, charging at 0.5C or lower is safer. Lipos like being charged slowly. You ought to be there while they're charging, of course, so you don't want it to take all week, but there's no point trying to do it as fast as you can - it'll just mean you'll be replacing them sooner, or possibly even risking a fire. If you need to leave the room, unplug it and finish charging it another time. That won't do any harm.
- charging to less voltage is better
The maximum you can charge a lipo cell to, without damaging it, is 4.2V. Above that voltage, the anode starts to build up a layer of plating that it will never lose, and the capacity goes down. If your charger allows you to charge to a lower voltage, it's better to do that. For one thing, there's always a margin for error whenever you measure something, so if your meter says it's 4.2V it might be 4.22V - which is too much. A lipo that's only ever charged to 4.15V per cell, or lower, will live a lot longer than one that's always charged to 4.2V per cell. At 4.15V, it's still holding 95% of the same charge but you'll get many more useful cycles out of the battery. At 4.1V per cell it's still holding about 90%. A slightly shorter flight duration, with a pack that you can use over and over again, has to be better than pushing it to the limit and constantly having to replace them.
Lipos are great for holding their voltage for weeks, so if you fly often there's no harm in charging them up as soon as you get home and putting them away ready for next time. However, if you know you're not going to use them for a few weeks, or longer, it's best to put them away without recharging them. A lipo with around 3.85V per cell or a bit less (usually after a fairly short flight) can be stored for a long time without losing capacity. A lipo that's stored for months with a full charge will lose quite a lot of its capacity, which is why they're never fully charged when you buy them new.
Last edited by Andy2No
on Fri Mar 07, 2008 9:57 am, edited 1 time in total.