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I am currently modifying a wireless video door phone system with remote door release system, for use with a walking frame, for my elderly mother in-law, who lives alone. The 7"monitor runs a 5v 1a DC plug pack normally, but I have used a 6v SLA battery and dropped the voltage to suit. (which works great) The problem lies when the battery voltage gets low around 6v, the voltage dropping circuit components reach very high temps while the unit is powered to normal use. (burning the components, causing smoke and blistering!) I have also setup a bi color LED to turn red at 6.1v, but is difficult to see and I can't seem to power a micro relay from it.

I was considering a low voltage cutout switching module, but 6v modules seems hard to get or to build with my limited knowledge (kits are one thing, designing and building from scratch is out of my league). Ideas are appreciated. 12v has been considered and is probably my next logical step, as 12v systems seem more available, but efficiency seems to go down and battery weight goes up (to use the same ah capacity). System use is 1A @ 5v DC, standby 300 ma. current battery size 6V 12AH (8 lasts a day) charge rate once a day (over night)

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  • \$\begingroup\$ A circuit diagram would help. Maybe you just need a diode from the SLA output to stop back current when the SLA is low. \$\endgroup\$ Oct 16, 2014 at 13:14
  • \$\begingroup\$ I have tried a few different approaches, so 3 circuits basically give me the same result, the current one actually has 2x 3A diodes, which also reduces the voltage after the low dropout voltage reg, - with a .22 cap on the inlet and 20 cap on the out - as recommended by the manufacturer of the reg, but still reaches 145 deg C when trying to draw 1A from the discharged 6v battery (@ 5.926V) current was actually measured by a clamp meter @.79A and the monitor became unstable with noisy jumpy video output. assuming that it couldn't draw enough amperage so was heating up and frying the components. \$\endgroup\$ Oct 17, 2014 at 11:19

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It seems I may have found a solution ( not fully tested as yet but looks promising).

I used a Mosfet based low battery cut off controller and "resistor-ed" it to suit a 6v SLA battery. I researched a Jaycar Electronics Kit ; KC-5523 (called Battery Saver) and combined with a bi color LED indicator module kit (also Jaycar) I have a fully functional unit which switches off @ selectable discharged state. (6v)

Components are: a charged 6V SLA , powering the 7" monitor / door phone, via a 5v LM2940 LDO regulator (.47 cap Vin side and 22 cap on the Vout). The Mosfet controller, the battery indication LED circuit, a 600ma 6v charger (which when plugged in disconnects ground to Monitor - shutting off power) As the battery is in low charge @ around 6v, it still functions on standby, but if the doorphone is used in normal power mode @ around 1A, the current exceeds the battery discharge rate, quickly reducing voltage and activating the mosfet switch, cutting power to the unit and saving an overheat situation with possible fried electronics ( I did install a big heat sink on the LM2940 just in case , and vented the case)

I havn't tested how the charger reacts as yet, hopefully it will function normally and when charged, will reset the mosfet switch ( requires around .5V above cutoff approx) I don't understand the Hysterics, but @ .5v I am assuming cutoff history is the point of reset. ... hopefully the project will be complete with some tiding up. thanks to those that had thoughts on the project.

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A buck-boost switching regulator would work best. It would regulate down when above the output voltage, regulate up when below the output voltage, and have a pass through region when the input is close to the output voltage.

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  • \$\begingroup\$ ill look into it in the morning. need to to do some research as I know nothing about this, but sounds like a good place to start \$\endgroup\$ Oct 17, 2014 at 11:14
  • \$\begingroup\$ I researched the buck-boost reg, but although it will give me appropriate voltage, I would need to integrate a drop out value ( Variable resistor?) for required cut off. or go down the IC path and program and set up a program - which is beyond me at this point. using a buck controller would give me the reliability, (and step down value) but seems complicated to control. \$\endgroup\$ Oct 18, 2014 at 12:30
  • \$\begingroup\$ Also looking at Mosfet control switch which seems simpler and can be controlled by resistors to give me a switch off point... thoughts please \$\endgroup\$ Oct 18, 2014 at 12:32

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