Fixed Wireless Technology

Batteries

While members of the ISP-Wireless list discussed rechargeable batteries, we talked about the history and maintenance of rechargeable batteries with the only battery services corporation we know of.

by Alex Goldman ISP-Planet Managing Editor [June 5, 2006]

If you're running a WISP, the first piece of information you need to have is whether or not any particular site has power. On the ISP-Wireless list in April, AT posted a simple monitoring method:

We use the tactic of putting dumb but pingable devices at the site but not connected to the UPS. When the device is no longer pingable, we know that power is out and an alarm is raised after a configurable number of minutes. (We use Nagios for monitoring—very good program!)

Old Linksys routers, broken DSL routers, etc. make great devices for this type of monitoring—lower power consumption and it saves them from going to the landfill.

We decided to find out whether there might be a better way to monitor the power on a network.

But before you decide how to monitor your batteries, you have to buy them. Luckily, the discussion covered that too.

AGM vs. gel cell
The discussion on the ISP-Wireless list focused on AGM versus gel cell batteries. Each has its advantages.

Having recently talked to a company that specializes in batteries, we decided to talk to BatteryCorp and see what kinds of batteries the company prefers (since they make their own) and what a top of the line battery monitoring solution can provide.

Steven Dworkin, BatteryCorp Founder and CEO, told ISP-Planet that there have been three generations of lead acid batteries over the past 20 years or so. The first generation was wet lead acid batteries. The second generation was made using a gel to immobilize the acid inside the battery. In the third generation, they took out the gel and immobilized the electrolyte in a honeycomb (the honeycomb is often called a "mat", hence the name, Absorbed Glass Mat, or AGM).

Dworkin said that companies prefer the suspended electrolyte technology for two reasons:

1. Companies are able to put more capacity in each battery. Because AGM technology takes up less space in the battery housing, battery manufacturers can put more capacity in a equivalent size battery. For example, given any two batteries of the same mechanical size, an AGM battery might have 7.2 amp hours of capacity while a gel battery might have on 6.0 amp hours.
   
   
2. Manufacturing costs are lower. Dworkin says that there are fewer flaws in the manufacturing process, and that using a glass mat separator provides more uniform results than a gel that can move around. When you throw away part of what you make, as you must when the gel process produces flaws, that adds to the manufacturing costs, raising the unit price of the product.

However, Dworkin said that neither technology is definitively superior. "People claim certain things, but I have not seen documented statistical facts proving one type is better than the other."

Instead, each technology is useful in its own environment. Dworkin said that the gel cell technology seems to withstand temperature variances, and is popular in the cable TV industry, whereas telcos, who generally have a controlled environment, tend to prefer suspended electrolyte batteries.

BatteryCorp itself sells only AGM batteries, but that's because suspended electrolyte is what the company's customers prefer. "We have the ability to manufacture gel cell batteries, but none of our customers has asked for it," Dworkin said.

Monitoring
So now you've chosen your batteries, one way or another, how would you monitor them if you were a BatteryCorp customer?

"There are a variety of technologies for the remote monitoring of batteries," Dworkin told us. "One technology is to monitor each cell of a battery, another is to monitor just portions of a string. You take data in real time, or weekly, and e-mail us the results of the monitoring and we track the trends and predict failure rates."

BatteryCorp doesn't monitor raw power. Instead, it monitors metrics such as impedance, conductance, or resistance on the battery and compares that with previously recorded data from the same battery as well as with the expected trendline for that battery type. When you get deviations from the baseline, you have a problem and need to replace the battery.

One of BatteryCorp's advantages, as a service provider, is that it has a great deal of historical data and has built that data into its software.

Charging batteries
Batteries can die early due to problems in charging. Dworkin said there are three types of charging errors:

1. Overcharging
2. Undercharging
3. Battery imbalance

1) When you overcharge a battery, you put too much current and voltage into it and as a result the battery heats up. Since the battery uses liquid, the liquid can evaporate, which can cause the battery to dry out. It can shorten battery life and, in extreme cases, destroy the battery.

2) When you undercharge a battery, it doesn't have the capacity you're expecting, so it won't run as long as you expect—or need—it to. Undercharging does not, however, destroy batteries.

3) Battery imbalance occurs when some batteries in a UPS receive more power than others. "If you have a string of forty batteries in your UPS, but the last two or three or five are not getting any power, the UPS will not last as long as you expect it to," explained Dworkin.

—End