R/C Naval Combat

I'm looking at going with NiMH type batteries in my Salt Lake City. how many milliamp hours should I be looking to cram in there. Also, what's the recommended set up for a 10 pound(ish) heavy cruiser motor wise? Should I go with 1 motor through a gear drive to power both shafts, or run separate motors? What size motors are recommended for each configuration?

Thanks

A 10K ton cruiser typicaly uses 5-7 amp hours. With SLAs the 5 amp hours are nice and square, easy to balance. Powersonic makes a 6.5 amp hour that is also square. The 7 amp are heavier on one side of the ship then the other.
Since you are using NIMH you can get almost 2x the power for the same weight or the same power for 1/2 the weight. You get to change batteries so you don't need 10-14 amps. Look for D or C cells that give you 5 amp hours. They are sometimes listed as 5000 miliamp hours.
Motors for a CA should be the 380 size. You don't need gear boxes if you don't want them. You can use direct drive and drag disk to make speed. I use 3 blade 1 1/4" props.

So about the same for a 3.5 unit predread?

The predreads should be heavy enough to run a 6V 12AH with no sweat.

http://www.cheapbatterypacks.com/?sid=1087373&pgid=loosecells&chem=NIMH

you might consider several smaller packs to allow you to better ballast rather than one large one. The 1950faups are excellent cells, so are the GP2000. the 4000 sub C cells should run you very nicely as well.

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Originally posted by Tugboat

The predreads should be heavy enough to run a 6V 12AH with no sweat.

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Kilkis? 10lbs or so I think.

Kilkis at 10lbs is a little lighter then my cruiser. a 12ah battery would be too much. A 5, 6.5 or 7 would be OK.

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Originally posted by Bob

Kilkis at 10lbs is a little lighter then my cruiser. a 12ah battery would be too much. A 5, 6.5 or 7 would be OK.

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I was off a bit. Max weight for Kilkis is 11.59.

You could go with a D sized NiMH pack. A 5 cell pack only weighs around 28 ozs (1 lb, 12 oz) and gives 10 ah. More capacity than a 6v/7ah lead acid at a pound less weight. The downside; cells cost around $6 each off eBay.

Ahhh... Kilkis. Didn't realize you were building one of the small predreads. Sowwy. :) Mike's suggestion with the NiMH D-cells is spot on for that.

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Originally posted by Tugboat

Ahhh... Kilkis. Didn't realize you were building one of the small predreads. Sowwy. :) Mike's suggestion with the NiMH D-cells is spot on for that.

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Yep. She's short and dumpy. Should turn well.

Sorry it took me a while to get back (Hockey playoffs have distracted me somewhat from RC Combat). Thanks for the responses.

Be warned: from my experience with nimh cells, they don't like being submerged, and when cooled rapidly by submersion can ingest water, which kills them, pronto. (plus they corrode internally sometimes)
If at all possible, I recommend the use of a waterproof container for them.

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Originally posted by Darren Scott

Be warned: from my experience with nimh cells, they don't like being submerged, and when cooled rapidly by submersion can ingest water, which kills them, pronto. (plus they corrode internally sometimes)
If at all possible, I recommend the use of a waterproof container for them.

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Thanks Darren. I would not have thought of that.

That's something I had not heard of either. Any waterproofing advice?

I've seen an entire battery pack tool dipped for protection. Personally, I went with liquid tape on the ends of each cell for protection.

I used tool dip on 7.2v sub-c packs in the past. I found that flexing the wires at the end would cause the tool dip to crack and fall apart. In my more recent packs I used superglue to cover the openings at the top and bottom, and that has worked fairly well. In future packs, I may use skotchkote instead. Skotchkote is similar to liquid tape and tooldip, but is much more flexible, stronger adhesion, and overall more reliable.

In general for waterproofing electronics I use 3M ScotchKote as my non-permanent waterproofer.

Put 3M Scotchkote on the positive end of the cells which will waterproof the vents. This is an important safety thing, since the vents can rust shut otherwise. Why the concern on the vents? Well, those are there to release pressure in case of the cell getting too hot. What potentially can happen if a cell's vent rusts shut is it has the potential to be a small grenade. DO NOT use Liquid Electric Tape as it will delaminate without giving any physical indications. ScotchKote will bind on to the cell, and is thin enough to get into the vents, but when push comes to shove and it's time to vent or go boom? The ScotchKote will still push aside and let the cell vent. I recommend 2 or three coats prior to soldering the cells together. Or if in an existing pack, you can just barely get the brush just in between the cells of the battery. If it's an existing pack or cell that's been sunk a few times, spray the positive end of the cellswith CorrosionX a few times to loosen any potential rust hiding in the vents prior to coating with Scotchkote.

Cheers
Jeff

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Originally posted by crzyhawk

That's something I had not heard of either. Any waterproofing advice?

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Reading through the thread, there is a lot of indications about using NIMH batteries without regard being given to the "C" rating (C rating is the Discharge rating, where C is the capacity of the battery. A 3000mah cell that has a 10C rating can discharge at 30A, whereas a 5000mah consumer cells may only be rated at .5C or 1C). Putting consumer cells inside of an old BB with efficient motors won't take that much, but a cruiser will take higher rated batteries. For my Des Moines cruiser running at Big Gun speeds, I've got 4 parallel 6V 3000mah 10C rated packs combined with efficient speed400's tied to gearboxes. Max discharge with all that is technically 120AH which is inline with SLA at that point. Weight wise, I've got 5 of those packs total, and they weight about a 1/3 less than the equivalent weight of SLA's. 15AH in high discharge NIMH packs comes out to about a little over half the weight of 7.9ah of SLA. I do build my own packs though so I can get packs custom fitted into the space. Commercial packs are typically built for cars with a customer "slot" for that long pack, rather than pre-dreads or cruisers which are a lot more cramped. I build the packs in a "pyramid" fashion which keeps the space utilisation down lengthwise, and pushes it up a little on height. I've gotten now where I'm using Scotchkote to "glue" the cells together and dispensing with the shrinkwrap. :) Anyway, there's a quick battery brain dump.

Cheers!
Jeff

I know a guy who is switching from lead acid batteries to NiMH cells. Just like Jeff above, he is going to stack 5 cell packs in parallel to get upwards of 30 amp hours and will handle 90 amps of draw ... at about half to 2/3rds of the weight of the lead acid batteries he was using before.

The downside? It'll cost 2 - 3 times more than the lead acid batteries.