H.U.L.C. – PRE-HEATING BATTERIES: MYTH OR POWER-BOOSTER?
It's almost like religion … Even before our two 16s Gaui X7 Megatrons had been completed, the discussion emerged whether or not a battery pre-heating to 40 °C (104 °F) was necessary.
No doubt this would increase performance as the chemistry inside becomes faster, but it has lots of disadvantages, too.
Again keep in mind we're talking about international logistics here! Space limitations, weight limitations and packaging problems are what make our life difficult. A voluminous styrofoam box (which isn't too stable either) certainly is among the last things you want to carry in your baggage.
Also, it means even more electronics plus additional battery packs to power its heating system. And hey, these things are badly energy hungry! Did you know? Putting your batteries onto the radiator for a while does not do the job.
Because your batteries do not get warm evenly. It may work for 3s packs – but certainly not for 16s. So you really need a closed box with a constant temperature. And in this environment you need to store your precious packs for about 3 hours. That long, really? Yes, that long.
During our first tests in Germany with temperatures below freezing point we had no choice but to use a heating system. With Spain in mind we designed a collapsible, transportable box – actually to our knowledge this had not been done before. As you can see from the pictures when collapsed it is quite flat, the electronics being stored on the outside. Put some bubble wrap around the whole thing and you can carry it on the plane without damage. It's still big, mind you. But WAY better than transporting a rigid box. After arriving in Spain
we simply clicked it back together; matter of 10 seconds. Then put the electronics back in and you're ready to go. That's one minute later :-)
The real problem is that it draws so much current and you have to exchange the heating batteries (5000 mAh!) every 30 minutes at the latest. Therefore, we first used our switching power supplies to bring the batteries up to temperature and only then changed to heating batteries to be mobile. For those who read the earlier post about our super-tight charging schedule: How/when do think this additional step was integrated? Yeah, right … NIGHTmare …
Well, you say, that sounds like a lot of effort. Do you really need this whole pre-heating rigmarole? Surprisingly, the answer is NO.
Normally you'd expect the batteries to be the weakest link in the chain. But the performance of the SLS packs (Stefans Lipo-Shop) was so good that under the Spanish sun (which rarely came out) at temperatures of 15 °C (60 °F) there was no need for pre-heating. This made logistics much easier for us, so after initial test flights we simply skipped the pre-heating step. So the bottom line could be something like this:
You want 10 kW of ultra-power? Then don't bother about pre-heating unless temperature is really low or it's no burden for you. Instead, go and buy the right batteries! Quite a learning that surprised many people around here. More interesting stuff coming next! #HeliGraphixHULC #HeliGraphixSpain #MissionSpain #HULCproject #MannedRCflight