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Old 09-30-2004, 12:40 PM
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Bill Bill is offline
Join Date: Feb 2003
Location: The mountains of Scottsdale, AZ, and the beaches of Maine
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Properly charging a battery, and then keeping it charged without overcharging it, is a fairly complex process. Starting with a somewhat discharged battery, there are three steps (or stages) involved in doing it properly.

Stage 1 - Constant charge current. The charger pushes a constant (and rather hefty) current into the battery, and monitors the battery voltage. As the battery accepts charge, its voltage gradually rises. When the battery voltage reaches a preset trigger point, the charger cuts off the high current, and goes to Stage 2.

Stage 2 - Constant charge voltage. In opposite fashion from Stage 1, the charger maintains a constant battery voltage (typically 14.2-14.5 volts), and monitors the current entering the battery. As the battery continues to charge, it accepts less and less current. When the current drops down to a preset trigger point, the charger stops, and goes to Stage 3.

Stage 3 - Constant but reduced battery voltage. The charger reduces its output voltage (typically to 13.7 volts or so) and holds it there. Because of the lower voltage, the charge current drops to zero, but as the battery slowly self-discharges, the charger detects this and drips in just enough charge to make up for it. If there are small parasitic loads that discharge the battery (such as the gas detector in the TM), Stage 3 will drip in enough charge to make up for these, as well.

If you switch on a big load, such as lights or a refrigerator, the charger detects this condition, and responds by switching back to Stage 2 or 1 as needed.

The important thing about Stage 3 is that it has a lower charge voltage than Stage 2; this is what prevents it from overcharging the battery. This is the signature of a good three-stage charger. If it doesn't drop the voltage, it is just advertising hype.


A cheapie garage-type charger has only Stage 1 - it will cram a high current into the battery for as long as you leave it connected. If you leave it connected more than a few hours, the electrolyte will boil dry. This is accompanied by much fizzing and spitting of acid, and generation of explosive hydrogen gas. The whole process destroys the battery very quickly.

A more expensive garage-type charger has Stages 1 and 2. Because it cuts back the charge current, it won't overcharge your battery as fast as a one-step charger - but it will overcharge it in a few days. Complete destruction takes longer - but it will happen.

Incidentally, the Magnetek 6300-series converter that came with the older TM's has a two-stage charger. Stage 1 is kind of a wimpy - it will do the initial charge at only about 5 amps. On the other hand, the 6300 switches to Stage 2 based on battery voltage, as is proper. But contrary to common belief, the newer Magnetek 7300-series converter also has a 2-stage charger. The 7300 has a much more powerful Stage 1 - but it switches to Stage 2 based on elapsed time, not rising voltage. If you fail to buy the optional timer, it is just a simple 1-stage charger.

Neither of the Magnetek chargers has a Stage 3, and so they are not appropriate for long-term battery maintenance when the TM is stored.

For a look at proper long-term chargers, we need look beyond RV applications, to fishermen. They use an electric trolling motor and a deep cycle battery. They fish for only a few hours a week (or month), and the boat is unattended for the rest of the time. When the fishing is done and the fisherman gets back to the dock, he wants to plug in a charger that will do two things. 1) If he decides to take tomorrow off and fish some more, the trolling battery should be fully charged and ready to go. On the other hand, if he doesn't get a chance to go fishing again for a month, he still expects that the battery will be fully charged and ready to go - but not destroyed. Most good 3-stage chargers were designed for the fishing crowd, not the RV crowd.


With regard to inexpensive "float" chargers or "trickle" chargers. These things are often very unsophisticated, as suggested by the price. They don't even have a Stage 1. If you start with even a partially discharged battery, they can't charge it - they just don't have the oomph to do so. And they don't have any voltage or current sensing - they just push out a small but relatively constant current. They rely on the fact that they don't have enough capacity to overcharge an average battery very quickly.

If you have a small battery, they can be trouble - I have had a trickle charger overcharge my motorcycle battery over the winter, destroying it. On the other hand, if you start with a fully charged average-sized battery, they will keep it more or less topped up, though not with any precision. And if you have a big battery, they may not have enough capacity to keep it up. But I guess it is better than nothing. Over the long term (weeks to months), they are not sophisticated enough to "properly" maintain a battery - but if you start with a full battery, they will do a better/safer job than the Magnetek/Parallax charger built into older (say pre-2006) TMs.

Just for reference, in my TM I installed a fisherman's 3-stage charger, Guest Industries Model 2610. I bought it quite cheaply on eBay, and I leave it connected 24/7/365. In Stage 1, it charges at 10 amps, and then it properly goes through the Stage 2 and Stage 3 operations to keep the battery up. There are certainly other reputable manufacturers. By the way, I am not a fisherman - I bought it at the recommendation of HappyTrails, a member of this forum, and I love it. Thanks, Happy.


Post Script. You can think of the whole process as trying to precisely fill a water barrel as fast as possible, without allowing any water to overflow onto the ground. First, you blast water in with a fire hose. When the water level gets somewhere near the top, you turn off the fire hose, and turn on a slow garden hose. When the barrel is exactly full, you turn off the garden hose. Then, as time goes on, you monitor the level of the water, and as water evaporates, or escapes through tiny leaks, you top it up with a teacup whenever it is needed.
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