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plasmahunt3r
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Joined: 10/01/2016
Location: United States
Posts: 10

Posted: 11:02pm 22 Oct 2018

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I have a need to isolate a remote battery and charge it from my regular charging system.

For example, I have a sailboat, and I would like the VHF radio to have it's own battery so it is always available for emergency call outs. I would like to use the existing low current wiring and circuit breaker for sending a reduced current to the remote battery. If I wire the remote battery directly to the boats main batteries and alternator, I would need heavy duty able to cover the distance and the current available. Typical remote systems on a boat use a Voltage Sensitive Relay that turns on when the engine/alternator is running and has high amp contacts, which essentially connects the remote battery in parallel, and requires thick diameter battery cable.

I don't know if this is the best way to go about it, but I created a test circuit, using a NPN transistor to control the current supplied to the remote battery and tested it in my truck. With the engine running and the alternator charging the main battery @ 14.31v, the remote battery received 13.62v @ .43 amps. The transistor and heat sink remained cool to the touch. This is because I was using an 8 amp transistor (MJE13007) but was only using .43 amp. I got this result using a 560 ohm resistor which resulted in a gain of 17.2 (14.3v / 560 ohms * 17.2 gain = .43a). If I lower the resistor value to around 150, I would up the current to the remote battery to 1.5 amp, which at 13.62v, would be fine to trickle charge any remote battery. The diode prevents back discharge into the main battery bank keeping the remote battery isolated.

Attached is the simple circuit I used and the prototype I tested. There may be a better approach but this seems to work.

davef
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Joined: 14/05/2006
Location: New Zealand
Posts: 499

Posted: 08:43am 23 Oct 2018

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Only two comments:
- the gain of the transistor can vary widely unit to unit, but for a one-off circuit tweaking the 560Ohm resistor gets the current you want
- how much does the transistor DC current gain vary with temperature?

Looks a neat solution to your particular requirements.

plasmahunt3r
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Joined: 10/01/2016
Location: United States
Posts: 10

Posted: 10:57pm 23 Oct 2018

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I made a change to the circuit and retested with a surprising result.

I changed the 560 ohm resistor with a 180 ohm resistor. Then I put my test load on the receiving battery and drained it down to 11.89v before testing. My test load is a diesel glow plug that is a near short and the plug glows red hot.

Then I restarted the test. Initially, with the 180 ohm resistor, the charger produced 1.69 amps. I was happy with the expected results.

Then a surprising result. As the battery reaches full charge, the amps delivered reduced gradually as the battery voltage increased. As the battery reaches full voltage of 13.6v, the amps delivered was only .38 amps.

I am ecstatic. I could not have gotten a better result. A battery to battery charger, that tapers off as the battery reaches full voltage. There will be no way I can overcharge the battery. Wonderful.

Boppa
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Joined: 08/11/2016
Location: Australia
Posts: 814

Posted: 10:49am 24 Oct 2018

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Only issue I could see, is if the load is connected to the battery and has an earthed case that connects to the other batterys earth (admittedly not likely in a glass boat, but possible in a steel hull) you would bypass your transistor
ie on your circuit, connecting the right hand battery - to earth directly as well, if you get what I mean

Depending on how things are wired, its a possibility (say radio has 1/4 wave screwed to a metal mast crossbar and it has an earthed mooring light up there too, you could get a direct earth connection through the coax shield and the radio - supply via the mast and anchor light -)

Apart from that, sounds great

Edited by Boppa 2018-10-25

plasmahunt3r
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Joined: 10/01/2016
Location: United States
Posts: 10

Posted: 12:06pm 24 Oct 2018

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Thanks Boppa. I will consider that. Ideally, each load is isolated, except if the boat is sinking in sea water, which is electrically conductive. I am not too worried about the windlass or marine head, but the VHF is a consideration as you suggested. I need to figure out a test for that. Off hand, I think I could use the ohmmeter and check for continuity between isolated batteries. While the transistor emitters are all common, the battery negative post shouldn't show continuity. If battery negatives shows continuity, there is a problem.

I just checked continuity between the main battery and the receiving battery, and while the emitter and main battery negative post showed continuity (as expected), the receiving battery negative and main battery negative didn't show any continuity. I will check each battery when installed and look for problems.

The positives are only on while connected to the alternator and charging because of the VSR. The diode should provide positive isolation between batteries. The VSR will probably come on while plugged in at the dock because of the battery charger on the main batteries. When the main batteries reach 13.2v, the VSR will activate and start maintaining the isolated batteries. The VSR could also activate due to solar panels charging at anchor.

Boppa
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Joined: 08/11/2016
Location: Australia
Posts: 814

Posted: 02:23pm 24 Oct 2018

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No worries, its just something to watch, as you can get some unexpected links in real life...
I first found the radio issue when new batterys were put in a truck and when they reconnected the cb power leads, much smoke escaped- they had actually put it across the top 12v battery (24 to 12v) instead of the bottom (12v to gnd) one and the - wire on the two way was connected to 12v instead of gnd at one end, and to gnd at the antenna...

A rather expensive woopsie, the only fuse was in the 12+ radio line, so it all just smoked until the wire burnt through- all the way from the battery to the radio and on to the antenna.....

davef
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Joined: 14/05/2006
Location: New Zealand
Posts: 499

Posted: 09:37pm 29 Oct 2018

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plasmahunt3r,

Decided to have a closer look at this circuit as I have a situation where I need to charge 2 series connected 13V5 SLAs from a LiFePO4 26V battery system.

Using a Schottky diode, the SLA floats about 0.3V below the source.

Another bonus is that this circuit has an automatic LVD (low voltage disconnect). In your situation at 13.7V to 13.8V you should see the current drop-off dramatically.

Neat!

davef
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Joined: 14/05/2006
Location: New Zealand
Posts: 499

Posted: 02:42am 04 Jan 2019

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I found that having separate negatives for the two batteries was a problem as I wanted to sense the "source" battery voltage but using the "receiving" battery -ve as the reference.

If one just connects the two negatives together and using a PNP transistor with emitter to "source battery , collector to the "receiving" battery and base through a resistor to the common negatives you basically end up with the "dual" of the original implementation. And no "separate negatives issue", as pointed out in an earlier post.

Was there some reason to have the series pass transistor in the negative?

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