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.

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About this Entry

This page contains a single entry by Karen Nakamura published on December 30, 2008 11:44 PM.

New SLA battery rack was the previous entry in this blog.

Batteryspec.com -- two thumbs up is the next entry in this blog.

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