Electric Flight in Australia

 

How do I fast charge my cells?

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7 Cells is the maximum you can charge at one time from a 12 volt car battery with an ordinary charger. When you fast charge, a nicad pack goes up to about 1.7v per cell. So, a 7-cell pack will reach about 11.9v. Electric current will only flow downhill, so this is the largest pack that you can charge directly from a 12v battery without a special charger. It used to be convenient to stay with six or seven cell packs because of the availability of chargers of this type and that's why we developed a 7-cell glider competition.

At one time I was charging 30 cells in 5 packs of 6 cells each. The resultant hassle soon convinced me that there was no substitute for a charger which steps up the voltage, so a large pack can be charged all at once. You see, you must have about 50 volts to charge a 30 cell pack which comes up to about 47.1 volts at peak when fast charged.

The simplest charger for packs of more than 7 cells is a voltage step-up transformer (DC-DC inverter) with a resistance lead to cut down the amperage and a timer switch to cease the charging if you are distracted.

The fanciest charger is a fully automatic electronic type that does everything for you, including selecting the appropriate charge rate for most battery packs. Most chargers are now like this, except if you are charging LiPo packs you MUST specify the correct number of cells.

A DC-DC inverter takes in the 12v from a car battery, and steps the voltage up to, say 42 volts. The voltage does not really matter, as long as it is higher than the receiving pack. What matters is the amperage. More of this in a minute. First, let me tell you that there is a cost. If you step up the voltage three times (to 36 volts), and you charge at 1A, you are really drawing something like 3A from your car battery. So, you really need a high amperage, deep cycle car battery if you intend to charge large packs. When charging 27 cell packs we found that we could only obtain about 3 or 4 good charges out of one car battery before re-charging the car battery. We were really drawing about 20 amps out of the car battery.

Now, on some chargers you do need to select your charge current. How do you select an appropriate current rate? Well, it really depends on the pack that you are charging. Normal fast charge nicads or NimH can be charged at about 2 X the nominal capacity of the pack. So, if you are charging 600AA cells of the normal type (like those in many transmitters), you could select charge rates up to 1.2A and take half an hour to charge.

The most common and safe rate to charge your LiPo cells is 1C - or one times the capacity of a pack. In other words, if you have a 2200-3S LiPo pack, that is 2200 mAh or 2.2 Ah, so to charge at 1C you charge at 2.2A. At that rate it will take 1 hour to charge your pack if it is nearly flat. Modern LiPo packs claim that they will accept faster charge rates up to 6C. They will take it, but you will probably shorten the life of your pack by continually using a fast charge rate. My advice is, use a higher C rating if you need a pack charged in a hurry, but stick to 1C when it is convenient to prolong the lifeof your pack.

One word of warning - you MUST charge your LiPo packs by selecting the correct voltage - ie. select 2S for 2S packs, 3S for 3S packs etc. If you do not choose the correct voltage, you could end up with smoke and flames. Most modern chargers will warn you if make an incorrect selection, but don't take any chances! So - there are two factors; the charge rate by which you select the amps at which you are charging (and it can vary) and the charge voltage, which you must get right!

If you are charging ‘R’ rated nicads (the temperature cut-off variety), you can charge up to three times the nominal capacity of the pack. If you are charging CP1300 SCR cells you can charge at up to 3.9A and take about 20 minutes. Some of us exceed these rates, but this usually shortens the life of a pack.

If you trickle charge your nicads or NiMH packs all the time, you can expect about 1,000 cycles out of a pack. If you fast charge all the time, you can expect about 400 cycles out of a pack. The best advice I can give is trickle charge a pack when it is new (at 1/10 rated capacity of the cells or less), and then fast charge it at the field. With continual fast charging, the cells will eventually get out of synchronisation, with some cells being higher than others. If you completely flatten a pack in this state, you risk driving the low cells into reverse polarity. Because of this, I do not recommend completely flattening battery packs in flight. I recommend that you give your cells a trickle charge, for 12 hours or more, from time to time. If you want to check whether your pack is out of synch, remove the heat shrink and use a digital multi-meter to check each cell individually. If you notice a variety of voltages, it’s time to trickle charge.

Some chargers, like the Hyperion and Schulze, have a discharge facility. You can also discharge and cycle your packs using a charger like this. If you flatten your cells by running them down in your aircraft, switch off as soon as you observe a noticeable “falling off” in power. The cells are now flat – do not go further. The same applies for running your motor during flight – switch off as soon as the power drops off.

One last thing - modern chargers also have a "Storage" function for LiPo packs - if you select that mode, the charger will bring the pack up to about 50-60% capacity and switch off, or discharge the pack to that level. That will prolong the life of your LiPos if you are not going to use them for some time.