Batteries / Power Systems: December 2008 Archives

I hate SLAs....

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I stopped riding about a month ago because I had severe sciatica. It's getting better now and so I thought I'd get back on the horse. One thing I had noticed last month is that my Piaggio was barely making it to work where previously it had been able to go there with energy to spare. I thought it was simply the cold weather.

For a related project, I bought a relatively sophisticated balancing charger, the Turnigy Accucel-8 150W 7A Balancer/Charger. One of the reasons I got this particular unit was that it could charge up to 36 volts of SLA; 32.4 volts of NiMH; or 8 cells = 29.6 volts of LiPo. Most other intelligent chargers I've seen can't handle this high a voltage. I want to experiment with switching over to NiMH or LiPO but I didn't want to have to take the packs apart to charge them.

One of the interesting aspects of the charger/discharger/balancer is that it has an accurate ampere-hour gauge, so you can tell exactly how many Ah you are putting in -- or taking out of the battery. I used this to test the various batteries I have in my stable and found quite a few bad packs.

What surprised me, though, was that one of the SLAs that I had been using in the Piaggio had gone bad. My original setup was 2S2P or two parallel sets of 2 serial 12 volt batteries:

 Battery 1: Rhino SLA17-12  (rated 18Ah) = 10.508 Ah @ 2A discharge
 Battery 2: Rhino SLA17-12  (rated 18Ah) = 10.485 Ah @ 2A discharge
 Battery 3: Tempest TR22-12 (rated 22Ah) = 12.092 Ah @ 2A discharge
 Battery 4: Tempest TR22-12 (rated 22Ah) = 1.880 Ah @ 2A discharge

Yikes!!! Very bad. And I had used it for less than two months... But no wonder I wasn't getting any mileage. I was going to just replace the battery with another similar sized one when I decided to see how many Ah were in the larger batteries that were in my power scooter.

 BigBatt #1: Tempest TR35-12 (rated 35Ah) = 30.612 Ah @ 2A discharge
 BigBatt #2: Tempest TR35-12 (rated 35Ah) = 30.539 Ah @ 2A discharge

Wowza..... This blew my mind a little.

  • Revelation 1: The big batteries were getting significantly closer to their nominal ratings than the little ones.
  • Revelation 2: Two big batteries had more amp-hours than four little ones == 30 Ah @ 24 volts vs. 22 Ah @ 24v
  • Revelation 3: The big batteries were 11.8 kg each x 2 = 23.6 kg
  • Revelation 4: The little batteries were 6.8 kg each x 4 = 27.2 kg

This all added up to .... it was better to use two big SLAs rather than four little ones because: 1) I would gain 50% more amp-hours; 2) it was lighter; 3) it was smaller; 4) and in hindsight, it would have been much cheaper.

That was easy enough to accept but it meant that I would have to redesign the battery carrier system. This was something I had been planning on doing anyay. More posts to follow.

I didn't like the wooden saddlebag style racks on my Piaggio that I had originally made. The problem was that the Piaggio didn't have adequate mounting brackets for them, which meant they were putting strain on the rear fender. Also, they interfered with the pedal action.

I decided to change the mounting system to a rear rack style. I removed the elongated double seat and replaced it with a bicycle seat. I used my MIG welder to bodge up a rear carrier, seen below. I think it looks pretty good.


More postings to follow re: the decision to change batteries and the actual battery carrier itself.

About this Archive

This page is a archive of entries in the Batteries / Power Systems category from December 2008.

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