Zener-lamp regulator

Lee Hart wrote on 2/26/03:

Here is a design I have been playing with for about a year. I have two working systems installed, and while not as aggressive as a full-blown Battery Management System, it does most of the job and at very low cost.

Battery regulators

Here is the wiring diagram of the battery regulators:

                                                  Battery -
                                                  5/16" 6 gauge
                                                  ring terminal
                                               ______________
                                              |     \    __  \
                                 _______________|/|__\  /  \  |
 Battery +                      |               |\|  /  \__/  |
 5/16" 6 gauge         ____|__  |  __|____    |_____/________/
 ring terminal        /    |  |_|_|  |    \
   ______________     \____|__|   |__|____/    zener diode
  /  __    /     |         | |     | |         1N5338B
 |  /  \  /__|/|_____________|_____|           5.1v 5w
 |  \__/  \  |\|
  \________\_____|        2 lamps in parallel
                          each #PR2

Charge Current Limiter

One simple way to force a charger to limit its output current is to put some resistance in series with its output. A light bulb is a good way to do this, because it acts like a crude constant-current source. The current only changes about 2:1 for a 10:1 change in voltage. For example, an ordinary 120vac 150w light bulb draws 150w / 120v = 1.25 amps at 120v, and about half this or 0.625 amps at 12v.

Wiring diagram:

                     lamp
                  120v 150w
                      __
                   __/  \__
                  |  \__/  |
                  |        |
charger+__________|___||/__|__________battery+
                COM1 /|| NC1
            ______
hot________|      |
           |      |____
120        |      |   _| relay
vac        |      |   _| 120vac
           |      |____| coil
neutral____|      |
           |______|
    children's "night light"
  (turns light on in darkness)

Parts list:

1 - 120v 150 watt light bulb and socket (choose wattage for desired limiting current)

1 - relay, SPDT contacts to match charger max amps and volts, 120vac coil (for example, Potter & Brumfield T92 series, DPDT 30a 240vac contacts, 120vac coil, Mouser 655-T92P11A22-120, $10.50)

1 - children's night light with photocell that turns it off when it senses light. Replace its lamp with the relay coil

Locate the night light inside the battery box where its photocell gets exposed to light from any of the battery sensors. When no light is detected, the relay coil is off, so its normally-closed contacts short the light bulb and the charger delivers full power to the batteries. When the first battery regulator lights, it pulls in the relay, which opens the contacts and puts the light bulb in series with the charger, limiting its current.