Print

Please login or register.
1 Hour 1 Day 1 Week 1 Month Forever
Login with username, password and session length.

[kiwimodeller]

Hi, I have a vintage Aerokits Sea Hornet runabout 25" long. Rather than the original diesel motor when I built it up some years ago I fitted what I think is a standard, cheap, Johnston 540 motor and an Electronize FR15HVR controller. Initially it had a 7.2v Nicad pack out of an R C car. It went reasonably well but I wanted a bit more performance and went to a 9.6v 800mah battery. That went well and still does. I downsized recently from a 3 blade 1 & 1/2" brass prop that I first fitted to a 1 & 3/8 two blade plastic prop and she gets up on the plane well, looks really good while running, and with the small prop the run time improroved but is still reasonably short. I therefore thought that as the battery is rather old and has been over charged and overheated a few times I would lash out and get a new one. I bought a 9.6v 2100mah NiMh pack but I have never been able to get anything like the performance of the old Nicad pack. I have tried charging with a 240v 300mA charger and with a HiTech Multicharge - amatic which works off 12v. The voltmeter shows it gets up to about 10.7v but  the performance would be less than 2/3 of what it is on the old battery. Even on the bench the motor does not seem to rev the same and takes a while to build up to a maximum which is well below the revs it gets on the old Nicad. Any ideas on why this is so or what to test would be welcome. Would I be better to go back to the 3 blade prop and run less revs but more load or are NiMh just different to Nicads? Thanks, Ian.

[derekwarner]

Ian......courtesy of Batteries & Chargers on their WEB page ...

"Some newbies think - There's a flier with a 150 gram pack of 8x600 AE cells - I have some NiMh 600mah cells that are far lighter - I'll use those." Unfortunately it doesn't work like that - because 600mah NiMh won't allow the current to come out nearly as quickly as Nicads, and when it does come out it will come out at a lower voltage. Since power (watts) is a combination of current (amps) time Volts, the power getting through to the motor is less than may be required for the plane to fly well - or at all"

This appears to be what you are questioning or eluding to...... - Derek
.

[PMK]

It's no secret that NiCad packs do indeed deliver a heftier initial surge of power than that of their NiMH counterparts - even if the NiMH A/h capacity is the same or greater. It's very much to do with the internal structure of the cells. Nickel cadmium is chemically a million miles removed from that of nickel metal hydride, but the benefits of NiMH cells outweigh those of NiCads in many ways.

It's likely you will notice a vast improvement in your NiMH cells after they have been discharged and recharged several times -- performing the same (sometimes better) than your NiCad packs.

[malcolmfrary]

Can't remember where I heard it (either Wiki or a "man in the pub") but maximum current available with NiMH batteries is not a linear function of size/capacity as with NiCads, it was mentioned that a 10AH cell would give nothing like ten times the current as a 1AH cell.  However, capacity for size seems to change week by week, and it may well be that a physically smaller, but higher capacity cell cannot yield the same current as a larger, but lesser capacity cell.

[nemesis]

Hello, Have you noticed that the portable power tool makers have stopped using NiMh and gone on to Lipos.
Follow the trade users, they want performance, all the time. Something that NiMh does not seem to give.
                                            Nemesis

[kiwimodeller]

Knowing nothing about Lipos apart from the story that they blow up if you mistreat them I did not consider them before ordering. Most of the packs you see listed seem to be about 11 volts. What battery would do the job if I was to change and what would I need to buy in the way of charger etc? Thanks, Ian.

[mattycoops43]

They are much lighter, so go for the highest capacity you can afford. You don't NEED to run a Lipo esc, but it is recommended, as a Nihm/nicad esc will discharge the lipo too far, killing it, unless you bring the boat in as soon as you notice a drop in power. With a Lipo ready esc, you tell it how many cells you run, and it won't allow it to go below 3V per cell.

A fully charged 3s Lipo is over 12V so you ned to make sure everything else will cope with the voltage, but they give much higher current  so you might find a 2s pack will go great in that and will be cheaper to buy. A fully charged 2s will be running at 8.4V when freshly charged.

They don't blow up as long as you treat them sensibly, puncturing them is a no no. but if your worried, you can now buy batteries for RC Cars which are hard cased in a plastic block. Shorting them makes them go bang, but then it does to Nihm's too!

You MAY struggle to get a lipo ESC that is for brushed motors, most are doing the switch to brushless at the same time. it is worth doing if you want more power, as brushless motors are now very cheap and much more efficient.

Hope that helps.