Vehicle Battery Jogger-keep your battery fresh and young （4）

Published:2011/8/8 0:50:00 Author:Phyllis From:SeekIC

By Karel Walraven

FETs

We have listed several options for FET T1, but the choice here is not critical. It’s also unnecessary to make any changes to the circuit if you use it at 6 V. Still, in this case the preferred type for Tl is the IRF540. That’s because this type of FET works with a lower gate voltage than the FETs in the BUZ family. For the same reason, FETs designated as ’logic-level FETs’ are more suitable if you want to use the circuit at 6V, but they aren’t essential. In principle, you can thus use any desired type of n-channel power FET that can handle at least 20 A at 50 V.

You’re probably wondering how a FET rated at 20 A can handle a current of 40 A or more. To understand why this is possible, have a look at the ’Safe Operating Area’ chart for the BUZ10 (Figure 3). It shows how much current can flow continuously (20 A), and how much current can flow for a short time. As the FET only conducts for a few tens of microseconds each time, the maximum allowable current during this short interval is just under 100 A. This information con also be obtained from the ’Absolute Maximum Ratings’ in the dale sheet, which specify 23 A and 92 A respectively.





Sulphation

The capacity of a lead-acid battery gradually decreases during its service life due to normal wear and tear, for a variety of reasons. However, the aging process is considerably accelerated if an uncharged battery is stored for longer than a week at a temperature below 10 CC or remains unused (while charged) for an extended length of time. That’s because in such situations, a layer of non-conductive lead sulphate (PbSO-j) forms on and around the electrodes. This reduces the effective surface area of the electrodes, thus decreasing the capacity of the battery. This process is difficult or impossible to reverse by normal charging and discharging.

However, it is possible to break down sulphate crystals into lead and sulphuric acid by using high (or very high) charging currents. This method can be used to restore at least part of the lost capacity (see ’Lead-acid battery revitaliser’ in the September 2001 issue). Still, it’s better to prevent the formation of sulphate. One way to do this is to periodically apply a load to the idle battery. The circuit described in this article is especially suitable for this purpose.

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