An experiment with LiPo batteries in radio control

Christopher Edge

In Gazette Vol 18, No 3, I wrote an article on building a radio control unit in a diesel body and was pleased to receive a few favourable comments from members. I hinted at further experiments using 2.4Mhz units and thought I had settled on the Acoms Technisport unit. Sadly they abandoned the earlier, and cheaper, version for a slightly different unit, and it is now very difficult to get receivers for the earlier type. Finally the price has rocketed up.

When I need some more items I may seriously think about purchasing a SkyFly unit. This is made in China, but seems to have reasonable reports and is not too expensive.

I have never been really satisfied with using AA or AAA batteries. They are bulky and even in a diesel not always easy to fit in, and do not seem to give as much punch as I would like. The good thing about them is they give added weight to the vehicle. This was a problem for the radio control flying boys.

Then along came Lithium Polymer (LiPo) batteries and the helicopter modellers were thrilled to bits. I briefly mentioned LiPos in my earlier article, and I dallied with them for a while. I admitted to blowing up several of them and having an expensive bonfire as a result of which I rather lost interest in them for a while. I have been stirred to try them again because the batteries and chargers have been improved and developed. I looked on eBay and did a lot of research, there are numerous websites and forums to see what others are doing. So I thought it time to experiment once more. I have bought a charger and three batteries (see photos) that cost under £50, so let’s see what happens.

It is worth explaining the properties of LiPo batteries, and how not to set fire to them as I did previously.

First the batteries are small and light. The size becomes more critical when one comes to build DMUs and the like. Regular sized batteries tend to encroach far too much into the passenger compartment, while a smaller LiPo can be hidden under the assorted boxes and other stuff on the underframe. Secondly each cell in a regular battery gives 1.2 volts while a LiPo gives 3.7 volts per cell and the standard unit to power your loco becomes a 3S. This means you need three cells to make 11.1 volts and they are wired in series, hence the S. Also I get the impression that they give a smoother current; maybe wishful thinking.

In the battery description, the 3S is followed by two figures, such as 20C or 30C. These mean discharge and charge ratings, and as far as I can see we can ignore these.

From these figures we can work out running times. This is not critical for us, if our batteries run out the train just stops, unlike a helicopter in the air which expensively crashes. The main large figure (eg 1000mAh) is an indication of the capacity of the battery so basically the larger the C figures the more capacity in the battery, and the more expensive. Just take care not to run down the battery to under 3 volts otherwise it may be lost for ever, but there are actually ways of redeeming the situation.

As ever there is a myriad selection of wire joiners and you pay your money and take your choice. I still have not made up my mind as to my preference, but I am slowly falling into the JST camp because they are neater.

Looking at the power pack there are two plugs. The one with red and black wires carries the charging current to the battery. The other plug has a number of wires linked to the S number plus ones, and has a multi plug on the end.

This is used in balancing and fits into a dedicated slot in the charger (see photo). This means that when you press the button on the computerised charger it gives a list of something like this for a 3S; 3.98, 4.01, 4.01. These are the voltages of each individual cell, and in this case the first is not balanced with other two. The program in the charger will, in balance mode, sort this out for you giving longer battery life. When all is balanced at 4.2 volts it will bleep. It is really important to download the manual and other relevant articles from the net and to read them. There are also quite a few videos. Studying these will help prevent a LiPo unit igniting with unfortunate consequences.

So with new knowledge and new confidence I googled a lot to find two things. First, as much data as I could find and second to see what the market had to offer. I should perhaps state for those not familiar with eBay that it is not just an auction, the ‘Buy it now’ option is commonly found in these electronic areas. I didn’t wish to fork out too much cash, but wanted to give it a good go. I placed an order one weekend and it arrived on the Tuesday. I bought a Turnigy 1000mah 3S 20C-30C LiPo battery, a Turnigy Accucell 6 charger and some connectors to match all up. The total cost was under £40. Not too bad.

The battery connected to the charger on the right, and on the left a 12 volt supply; a car battery is fine.

I tried to take it slowly, set up the charger and made up some cables and joiners, fiddled around a lot, fitted a DMU up with the new battery and switched it on. The air was blue (literally) as smoke poured out. I didn’t panic, but I moved faster than I have for a while and shut the lot down. It looked like while stripping the wire for the battery connectors I had got over enthusiastic and stripped too much plastic from the cables and caused a short. One must take care as these batteries are very powerful and sensitive. I went back to Google and searched ‘Accucell 6’ and found full instructions in pdf form. Now I cannot state this advice enough – read these instructions thoroughly. You also can find other websites with videos showing how it works. Watch them, they are useful.

After a good check I calmed down. Luckily I had done no damage so tried once more. Taking great care, I tentatively threw the switch on the drive unit and the lights came on in the receiver. I moved the lever on the transmitter and the motor turned. I rushed out to the tracks in the garden and pressed the lever and the bubblecar picked up her heels and scampered down the track at a speed far superior to when she ran on 10AA batteries.

Shows at the rear of the chassis a balsa cradle for the battery and an elastic band to secure it in place Battery fitted Positions of RX and TX in a Lima 33

There some important principles in battery fitting. The battery cannot be charged in situ like a NiMH, because of the risk of a fire, so it must be fitted where you can get it out easily to charge it. The smaller lower amp/hr batteries tuck nicely behind the boxes on the underframe of a DMU. Another advantage of easily removable batteries is that the expensive LiPos can be swopped into other vehicles, helping with the economics.

There are downsides to these batteries. They are not cheap so look after them by reading the manual and other articles. I learned the hard way. Never leave a charging Lipo unwatched. Charging is the most dangerous bit as heat can build up, and if it goes bang, you’re in trouble. Yet again read the advice on the net. I make no apologies for repeating myself about this, but for once Health and Safety is important and if they are mishandled they are dangerous things.

You may find that the lead from the battery is too long and you cannot hide it. If you do shorten it, take care not to get a short. Do one lead at a time then all will be safe. After another near burn out I decided that the single strand wire I was using was not man enough and was overheating so I upped the wire rating.

After experimenting with various locos and DMUs using the new battery, and a lot of care to keep the polarity right, I reckon I shall settle on the following:

Give it a go and I think you will be impressed. I am.