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Old Apr 02, 2012, 08:35 AM
Got the Slickness
gabrielisrael's Avatar
Seattle, WA
Joined Jul 2010
2,521 Posts
Cute.... I like it!

Looking forward to seeing the finished product.

Question: Why are you not using lipo batteries?
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Old Apr 02, 2012, 09:21 AM
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Stuttgart, Germany
Joined Oct 2004
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Quote:
Originally Posted by gabrielisrael View Post
Cute.... I like it!

Looking forward to seeing the finished product.

Question: Why are you not using lipo batteries?
Hi Gabriel,

The battery pack is made up of lithium ion cells commonly used in cordless power tool packs. They are less sensitive to imbalance than lithium polymer, are less likely to explode, and are much longer lasting than lipo cells. This pack in particular is a 2S11P pack with a center tap for balancing, with a total capacity of 14.3AH, and weighs about a third less than 2x 6V 4.5AH SLA batteries. It has a 60A fuse for protection in the event of a dead short, but I added a 15A fuse to protect my ESC.

A full charge from empty would take almost 6 hours at 2.5A which is my chargers max rate. These batteries are rated for 0.5C charge rate so I could theoretically charge them in 2 hours if I had a charger capable of 7A charge current. However, this would defeat the purpose. I just wanted something lighter than SLA with better capacity, with the ability to take a charge at my Triton's max rate.

It fits my style of boating when I want to go out for a couple of hours run time at a moments notice and don't want to recharge after every run. And, these cells have almost zero self-discharge so they don't need maintenance or trickle charging in the off season. I can dust it off on the first warm spring day and go to the river with almost zero preparation time.

If you replace the first sentence of the previous paragraph with "It fits for when I want to work around the house at a moments notice and I don't always remember to recharge after every use", you'll see that my usage pattern is almost identical to that of the average DIY cordless tool user and these cells are engineered to do exactly this job.

If you think about it, SLA batteries are a terrible fit for model boats. Their energy capacity to weight ratio is awful, they can only be charged safely at a fraction of their C rating, and they only deliver their stated capacity if you discharge at a ridiculously low rate. They degrade with age, with frequent use, and with infrequent use. Heck they degrade just from looking at them too much.

The only reasons why they are popular for model boats is because they are relatively cheap to buy and boats can generally handle their excessive weight. But this doesn't figure in the fact that they are heavy to ship, and require periodic trickle charging to keep their charge. I suspect a lot of modelers end up replacing them long before their useful life should have ended because of concern about degradation so in the end I don't think they are as economical as they seem.
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