What battery to power a field charger

[buster prop]

I've always used my car battery without problems to charge up to 3300 3S several times during a session at the field. My car's an automatic so won't push start! Some internal 10A outlets in cars don't work with the ignition off, Luckily mine does but I made up a lead with a ciggie lighter plug at one end and 2 x 4mm sockets at the other to plug the charger into and the lead's long enough to reach outside of the car. A friend has a mobile hangar (Transit van) with a leisure battery in the back to charge his planes from. I'm sure you won't have any starting problems unless you charge a lot of very big LiPos for a long time. Incidentally I did read that you musn't start the cars engine as the voltage will go above what the charger can take.

[Stephen Grigg]

I normally carry jump leads and have on a number of occasions jump started the electric flyers who have used the batteries on there cars to charge there lipos.

[flytilbroke]

Most of the 12v input chargers are designed to accept a considerably greater input voltage. The figure of 12v is a loose description when applied to most consumer items especially when it comes to automotive items. Modern cars/vans have reasonably well regulated systems compared to the old "Generator" types of quite some time ago. It is quite easy to make a lead suitable for our charger connections using a heavy wire eg: light jump lead using a plastics box and electrical connector posts. Maplin and most suppliers can provide them. They can be built to provide multiple connection points. So, do not fear actually using the car as you can then position the charge site clear of it. It is very unlikely that a running car engine will provide a voltage which is too high and it will allay the fear of a flat car battery.

 

I have been using such a system for many years both for model use and Amateur Radio.

[Peter Beeney]

 

 

Like Stephen I too have always carried jump leads and I too have used them quite a few times. The last time was only about 2 weeks ago; and when it comes to cars like BMW’s, you have to be extra cautious, there are special ways of doing it. And like all batteries, many car batteries are not always in the best of health, some can be quite poorly.

One way of checking the availability of the power is to convert everything to watt hours and then you don’t have to consider the different voltages. So the 7Ah 12 volt will be 7 by 12 = 84Wh, watt hours. The lipo will be 2.2Ah by 11.1 volt = 24.5Wh. 84 divided by 24.5 = 3.4 times charged. However, you never get something for nothing so you have to add at least 10% to the lipo total, this being to drive the charger and other losses. So it’s now 84 divided by 26.9 = 3 times. This of course relies on your lead acid being in top nick, and they are not always in this state. You can only really try this in practical terms, lets say the lead acid is only 6Ah good and you have to add 15% for losses. So it’s now 6 by 12 = 72 and the lipo is 24.5 plus 15% = 28Wh. 72 divided by 28 = 2.5 times.

As an example, when my charger is charging at 156 watts it has an input of 204 watts, this gives it an efficiency about 76%. Some inverters can soak up the power.

If you use the battery for starting and glow etc. then this figure would rapidly get smaller.

I’d have thought that most chargers will have a maximum input voltage limit at about 15 volts, otherwise they wouldn’t connect to a PSU, the voltage of which is 13.8V, I believe, the exact ‘float’ charge voltage of a 12V lead acid. You can leave your 12 volt battery permanently connected to this.

I defeat my car’s computer, this wants to switch everything off in a random way, in a sneaky manner. I’ve always used a car battery for model starting, this is always connected in parallel to the car battery. I just backfeed the car’s electrics via the axillary power socket in the back, so the computer can’t shut down. At least the radio always stays on!

Maybe there m something of use here.

PB

[Braddock, VC]

I use my suv to tow my caravan. The electrics that support the caravan internals eg fridge remain live when the ignition is switched off. It's the grey one of the two female connectors and I've made up a lead with the grey male connector at one end and a cigar lighter socket on the other.

After several charges run the engine for a while.

[John Cole]

Why spend a lot of money on a 12V Leisure battery when you could buy some more LiPos instead?

[Ben B]

If you are charging your lipos using the car battery (with the engine off) you can use the chargers voltage input protection to prevent over-discharging your car battery and leaving you walking home.

Without any drain on the battery you should see 12.65v across the terminals when 100% full, reducing to 12.24v when 50% empty, and 12.06v when 75% empty.

 

I wouldn't take my car battery anywhere near 75% empty because I don't want it failing later or take a risk it won't have enough oomph to get start the engine. So I'd use 50% maximum.

 

I use the 7Ah field battery to charge my 2200maH (and smaller) 3S batteries and expect to get 3 charges off a fully charged 7Ah battery if only charging at a low C rate (1 or 2C). Charging faster pulls the voltage down and then I won't get more than two and a bit. The main time the 7Ah battery isn't man enough for the job when charging 4S batteries. Even with 4S 2200mAh batteries I only get a couple of charges (obviously the charger has to do the 12v => 14.8v by pulling more current). And if I try charging at 10 amps or so it will do it when fresh but it quickly drops it's voltage...

 

My way around this problem is by building a mini generator. A 200W generator can be really quite small (and about the same weight as 2* 7Ah batteries that I currently lug around).

[Chris Bott - Moderator]

Ben do you have any more details on "building a mini generator"? I'm sure many on here would be interested.

 

My issues with field charging have always been how long it takes. Sure some LiPo's will stand a 20 minute charge but many need a good hour or more if we keep charge rates within manufacturers limits. (Which we must for fear of real divine retribution if we don't).

So when I put one Lipo on charge, that charger is tied up for some time. Using multiple chargers requires an even meatier charging battery.

So with LiPo;s I tend to take enough charged ones for all the flying I want that day.

 

I do however use a lot of A123 flight packs. I charge these in about 15 minutes. This does indeed require a good battery. I did buy a small car battery but that only lasted a few months. Probably because I over did it's discharge too often.

So the best solution (apart from a generator) is the leisure battery. I have a 110Ah one that was sold to me as a dual purpose Leisure/Cranking battery. Intended for use in motorhomes or boats. This will apparently stand high discharge currents as well as deep discharge.

I do try to look after it. Charging as soon as possible after use, giving it a top up once a month and not letting it very cold.

The big disadvantage is it's weight. We have a good walk from car park to pits, so a good trolley is needed.

[Ben B]

It's still a work in progress but certainly when it's finished I'm going to put up a build blog.

 

Basically it's a 20cc McCullen petrol engine from a weedwacker (via Ebay) which puts out just over a horsepower (which is approximately 750 watts). This is linked up to a motorbike alternator which charges a very small moped / motorbike battery (which is basically just used as a buffer).

 

Even at 50% efficiency I should be able to charge my 4S packs at 10-15 amps.

 

The main issue is silencing. It's a 2-stroke and the stock silencer is very small so it's noisy. But I'm working on that.

 

In retrospect a water-cooled motor would have been better as I could have put the whole engine in a sound-proofed box with only the exhaust exiting and just had an external radiator for cooling. As it is I need air to cool the motor so I cannot do this.

 

Alternatively, something like this should do the trick:

 

[Chris Bott - Moderator]

Sounds great Ben, a build blog would be interesting indeed.

 

I can just hear the derision from the I/C guys now for using and engine to power our electric models

[kc]

Just when we thought noise problems could be a thing of the past someone decides to use a chainsaw engine to charge his electric flight packs! You are joking surely? It's an early April Fools isn't it?

Next thing is he will tell us how he plants sunflower plants around the flying field harvests them, presses into oil to power his generator engine then how he converts it to run on cooking oil...........

[Chris Bott - Moderator]

Told ya!

[Phil 9]

cut out the middle man and just bolt the engine onto the model. Now there is an idea

[Ben B]

It's no joke. Anyway, it doesn't have to be noisy. As it is I mostly fly electric but running out of battery packs is annoying and it'll cost $$$$$$$ to get enough to fly all morning when I'm pulling 120A out of them. The way I look at it, it's the best of both.

[Phil 9]

I have often wondered if there where any suitable solar chargers available as most flying is done outside on bright days. But I am guessing they are all to slow?

[Phil 9]

Maybe the solar idea is not that far fetched?

[Chris Bott - Moderator]

I think it would be possible Phil, there are some pretty powerful ones available for caravans/boats etc. But they would be bulky and very expensive.

[Ben B]

I looked at solar panels but to be honest it's not worth it. They weight a tonne. A 200 watt solar panel is B-I-G! And lets face it, how often does it actually shine in this lovely land of ours?

 

BTW- just checked a mere 19kg!  And that's 200W max not average.

Edited By Ben B on 20/12/2011 22:08:43

[Phil 9]

looking around some more its not an easy option. Maybe a small trickle charge panel on a flight box may keep a 7ah sealed battery toped up but thats about it

[Steve Hargreaves - Moderator]

Love the micro genny Ben.....a great project.....aeromodellers are an ingenious lot aren't they.......

 

As you say though pretty noisy.....I can imagine fields full of these all whirring away. Indeed we could see the situation where electric flying is banned because the charging is so noisy whilst IC model can continue flying......that would be ironic...

 

People often forget that electricity isn't an energy source in itself but a means of transfering energy......you still need a prime mover to create it either by burning fuel in a turbine or an IC engine.....

[Olly P]

Wind Turbine....next to the field...hmm maybe not!

[kc]

 

 

IC pilot " I cannot hear if my engines cut for the noise of the *******silent flight generators!"

 

Any time now someone will invent hand launched gliders............................

 

 

 

 

 

It's interesting to see what ideas have come up when someone asks a simple question!