Interested in NiMH scooter batteries?

[bmazin]

Gavin and Mako scooter drivers:

Are you interested in a NiMH battery pack for your scooters? I am envisioning a drop in replacement for the lead acid systems. The battery would be 24V 26 Ah, 26 lbs (you'll need to add a little lead to the Gavin to get it neutral).

The main benefits of this system would be:

1) Longer battery life. The pack would bring the battery life up from ~50 minutes to ~2 hours at full throttle on the Mako, similar gains on the Gavin.

2) Much longer battery life in the cold.

3) Longer cycle life - treat the batteries well and you'll get >5x the cycle life of a lead-acid battery. This helps make the packs more affordable.

These packs and smart chargers have been used for a while now on electric bikes, so the technology is pretty robust. I have used the packs quite a bit and I think they will be able to take rough handling far better than the plastic lead acid batteries currently used.

First guess at costs would be in the ~$800 range for the pack, and ~$75 for the charger. Let me know if you'd be interested in this system - if there is sufficient interest we could design and test the packs pretty quickly.

Ben Mazin
President, MST
ben@jetboots.com

 

[The Pirate]

I'd like to see more info on these proposed battery packs. Do you have a website with some more info?

The Pirate

 

[bmazin]

I'll put a website together this weekend. For now, here is some more info.

General info on lead acid batteries of the type used in Gavin/Makos can be found at:

http://www.cavediver.com/cdbatt.htm

Lead acid batteries have been the mainstay of large capacity rechargable systems for a long, long time. They are a very mature technology, but have some disadvantages when compared with the newer nickel metal hydride (NiMH) and lithium ion systems. What I am proposing is replacing the lead acid battery packs with a similar weight of NiMH batteries. Since the NiMH batts have ~2x the energy density, the replacement packs will burn for twice as long. NiMH batteries also have some nice additional features, like 500-1000 cycles lifespan, good cold weather performace, some tolerance for shorts, overcharge, and overdischarge, and good performance at high discharge rates. You can also fast charge the batteries (in as little as an hour if you are willing to shorted their lifespan). These batteries are seeing a lot of commercial use in electric bicycles and are performing well.

I will engineer a drop in solution for both bodies. I'll probably use 2 packs in parallel, each pack consisting of 20 NiMH F cells in series to give a nominal 24V. The packs are assembled with welded on nickel tabs that have a very low resistance. The packs are extremely solid - there will be no mechanical issues. I can send you the battery data sheet if you shoot me an email.

You'd simple remove the lead acid batts and insert the packs/holder into their spot and hook up the battery connecter, and away you go. I've attached a picture that shows in general what the packs look like - the packs for the scooters would be more square than those pictured, however. Each pack's outline will be a bit smaller than the Mako's Yuasa NPG18-2.

If you dive a lot these packs could actually cost about the same in the long run as you'd have two sets of batteries to match the performance and the lead acids have short cycle live, so you need to replace them much more frequently.

Let me know if you have any other questions.

Ben Mazin

 

[dmdalton]

Hi Ben,

I was thinking about the advantages of this a while back (have had my Gavin a year) and thought that the pack could be made shorter which would allow a reduction in length of the Gavin, and less lead balast resulting in a smaller and lighter scooter.

I only did a rough figure on it. What do you figure you could reduce the length of the pack by?

Dave D

 

[bmazin]

The density of the NiMH cells is about the same as lead acid, so if you wanted to keep the same burn time as the lead acids you could basically take off half the length of the lead acid battery (maybe only three inches). I think that a lot of the gavin is empty space to give enough bouyancy to float the batteries, so you'd be able to chop quite a bit more off as the battery pack weight would drop in half.

This would involve major work on the body of the scooter and considerable playing around to get bouyancy and trim right, so my preference would be to just go with the original plan of doubling the battery life and keeping the weight the same.

Ben

 

[Aviatrr]

Ben,

Have you built and tested a pack for burn time? I did some experimentation with NiMH about 6 months ago for the same purpose, but things didn't work out so well. I took two sets of 24v 8ah D cell NiMH packs and wired them in parrallel, then attached them to a burn tester. I ended up with about 42 minutes of burn time.....not anywhere near what I was hoping for. Based on those results, I decided not to drop $450 on batteries to build two F cell packs that may not increase my burn time more than about 20 minutes. I currently get 50 minutes out of the lead acid.....and I estimated that with 24ah of NiMH I would only end up with 65-70 minutes. Not worth it for the price difference.

If you come up with different results, let me know. I am waiting for a friend of mine to get me some new batteries to experiment with...but they won't hit the market for 6-9 months, and he probably won't get the "test cells" for 2-3 months from now.

Mike

 

[bmazin]

Haven't built or burn tested them yet - I'm trying to judge interest first.

I think what happened with your experiment was that your pack was rated at 16 Ah and you probably used the Mako/Gavin burn tester, right? This means that you dischaged the batteries at 1.3C or so (1C means you discharge the batteries fully in 1 hour). All batteries have a lower capacity when discharged quickly, sometimes quite a bit lower. Not knowing the specifics of the battery or cutoff voltage you used it is hard to say how much lower the actual capacity is at 1.3C, but it is probably quite a bit lower than the advertised 16 Ah (for NiMH the stated capacity is often measured at discharge rates much less 1C).

Also, if you're only paying $450 retail for 2 20 F cel packs, you may be getting some low quality batteries - there are some real differences between the different manufactures, especially at high discharge rates.

Using the proposed 26 Ah pack you'd be discharging at ~.5C, so you should get close to the theoretical maximum life. My experience with Jetboots has shown that the packs seem to conform well to the spec sheets.

Ben

 

[WYDT]

Hi Ben,

I thought of doing NiMh for a scooter myself a few months ago but that's kinda cost prohibitive.

If it's for a Mako and cave diving I'd rather just get another Mako and have real redundancy. You can find used ones for ~$900 - $1000. If it's for O/W diving most dives are less than 1hr anyway and extra batteries (SLA) are only $90 a set. I could get more than 70min burn time with the pitch turned down to 7 on a single pack.

As far as a new Mako goes if one spends $2000 then another $800 for the NiMh one may as well go ahead and get a Standard Gavin that can go three times deeper and already has 33ah burn time. The one I just ordered with AGM batteries is actually 35ah.

I just think until the price on these NiMh batteries come down it's not worth it... just my opinion.

Now if you could make a set at half the price you're talking about then you'd have something everyone may want....

 

[bmazin]

The economics acutally work out pretty well if you dive a lot (or rent the scooters out). Let say you do two scooter dives a day. You'd need one set of NiMH or two sets of lead acid to do this. The lead acid batteries last about 100 cycles, the NiMH 500 (conservative). This means that you'd need to buy 500/100 = 5 sets of lead acid batteries to give the same total lifespan as the NiMH. 5 replacements * 2 sets * $90/set = $900, a bit more than the NiMH.

Also, be very wary about the claimed capacities on the lead acid batteries. The 33 Ah for the PowerSonic PS-12330 is the capacity you get if you discharge the battery over twenty hours. At the discharge rate used in a scooter this same battery acutally has a capacity of around 19.8 Ah! See:

http://www.marvac.com/products/powersonic/ps12330_lg.htm

Ben

 

[WYDT]

So are you saying your 26ah NiMh pack would burn longer than a 33ah SLA or 35ah AGM batteries? How? Are they 26ah for a 1hr discharge?...

Not trying to be argumentative just trying to understand.

Economically speaking I'd rather spend $900 over 5 years (assuming 100 scooter dives a year wich is pretty high) than $800 up front. That impacts my wallet much less and if the scooter is flooded or lost/stolen I don't loose $800 extra bucks.

I still think the NiMh batteries are a good thing... how can more power in less space not be a good thing... they are just too expensive right now for the typical diver.

Thanks!

bmazin once bubbled...
The economics acutally work out pretty well if you dive a lot (or rent the scooters out). Let say you do two scooter dives a day. You'd need one set of NiMH or two sets of lead acid to do this. The lead acid batteries last about 100 cycles, the NiMH 500 (conservative). This means that you'd need to buy 500/100 = 5 sets of lead acid batteries to give the same total lifespan as the NiMH. 5 replacements * 2 sets * $90/set = $900, a bit more than the NiMH.

Also, be very wary about the claimed capacities on the lead acid batteries. The 33 Ah for the PowerSonic PS-12330 is the capacity you get if you discharge the battery over twenty hours. At the discharge rate used in a scooter this same battery acutally has a capacity of around 19.8 Ah! See:

http://www.marvac.com/products/powersonic/ps12330_lg.htm

Ben