Battery to Battery Charging

[plasmahunt3r]

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.

[kitestrings]

plasm'r,

I'm the the electronics guy here, but a couple questions:  Does the charging source run regularly, or for long enough time to recoup whatever load plus losses are on the aux. battery?  If I follow your math, you basically have about a (13.62V * .43A =) 6 watt charger here.  That doesn't seem like much capacity.

Is there a benefit to adding the transistor?  Will it potentially add any unwanted noise?  Most of battery isolators that I've seen are essentially just a diode and the source, presumably with its own regulation charges both batteries when available.

Regards, ~ks

[plasmahunt3r]

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.

TO Kitestrings:
The transistor is an amplifier, that amplify's the current applied to the base.  I want to control the amps delivered to the receiving battery.   By varying the resistor size, I can set the base current  and the transistor will restrict the current gain to the battery.  I have a need for several remote batteries, each with their own requirements.

The way I am intending to use this, is to set up a VSR (Voltage Sensitive Relay) that triggers when the alternator is on.  At least 65 amps will be available to the VSR.  The output from the VSR will go to a fuse box which will have multiple isolated and remote batteries hooked up to it.  One battery will be for VHF and I will probably charge at 2 Amps.  One Battery will be up in the bow to operate the windlass and it may get 5 amps.  One battery would be to operate the marine head to macerator pump and electrolysis to convert the sea water intake to chlorine to sterilize waste.  The Marine head battery will charge at 5 amps.  The marine head and windlass get occasional use but high drain when they are used.  I want to offload those loads from my main batteries, which operate the boat essential systems.  The only other batteries are dedicated to refrigeration, and the wind generator is dedicated to producing cold beer.  Solar will supplement the main batteries while at anchor.

[joestue]

seems to me you can get rid of the diode. discharging the source battery will reverse bias the transistor but you have a few volts to go before anything bad happens.

[plasmahunt3r]

The diode is essential.  In this circuit, the receiving battery would turn on the transistor through the resistor.

[OperaHouse]

I see no point to this circuit other than drawing excessive power all the time to drive the transistor. A very low value resistor in series with the diode would accomplish just as much.

[plasmahunt3r]

Thanks operahouse.  Your idea does seem simpler.  To get a 3 OHM 100W resistor would cost over $5.00 while the transistor is 40 cents.  I could get resistors from china for .99 but I would have to wait a month for delivery.

[MagnetJuice]

Here is a crazy idea. Crazy because not many people would go to the trouble of messing around with this kind of stuff. But here it is anyways.

Sometimes I need a small, hi power, low ohm resistor. If I don't have it, I build my own using nichrome wire from a junk hair dryer. I buy these dryers for about $2 at second hand stores.

They usually have wire of 2 different gauges. For example, this one has 24 Ga. and 28 Ga.



The 24 Ga. has a resistance of 0.14 ohms per inch. For 3 ohms that would be about 21 inches of straight wire. I straighten this wire by holding both ends with plyers and pulling on it. This wire can't be soldered, it has to be crimped.

I also have built a few foot warmer insoles using this wire. I like to warm my cold feet before going to bed after staying in front of my computer until 1 am.

I use the hair dryer in the picture to test small power supplies and anything that needs to be tested using a variable resistor.

Ed

[OperaHouse]

I'm thinking  a 1/4 50 half ohm. 

[plasmahunt3r]

I don't understand.  A .25 ohm or 1/2 ohm will have too much current and that would defeat my purpose of using the existing 14 awg wire.  I don't want to add a bunch of heavy battery cable across the boat. 

I have ordered a 5.7 ohm and 2.6 ohm resistor from China to test.  I am looking forward to testing and comparing with the transistor method.  I wonder if the resistor/diode method will be a static (constant) current or will it will taper off as the battery reaches full charge, like the transistor method.  I would like to have the current taper off to nothing as the battery tops off.

[OperaHouse]

Lets look at the function of each part.

The diode is forming half of a battery isolator. It may not serve any purpose. You can sit in the bay for a week and listen to the radio. That can drain both batteries so you can not start. It only protects the radio battery.  So, you can crank till the main battery is dead and still call for help. That may be worthwhile. Normally an isolator feeds the alternator current thru two diodes making both batteries independent.  If it is a SLA, a standard diode drop is useful in dropping the charge voltage and not boiling the battery.

The battery operates in a narrow voltage range, 12-14.2 volts.  So, 2.5V at best to drop in a .5 ohm resistor or 5A at best. Current tapers off as charged battery volts increase anyway with a battery. At your low currents I wonder if the battery can ever be fully recharged. And I would use some low voltage high current Schottky diodes in parallel to get the diode drop down as low as possible to get the proper absorb voltage.

That is my take. 

[plasmahunt3r]

The power consumption and replenishment on a cruising sailboat is a tough problem.  Battery space, location, and charging is a problem.  I would like to have one massive battery bank that I can tap in to, but the realitiy is, that I can fit 2 main batteries near the engine, I can fit a couple under the galley sink, a battery in the nav station, one under the head sink, two in the v berth.  Cruising consumption may mean 172AH a day for 8 days to get across the Gulf of Mexico.  I would use considerably less power at anchor and can catch up on my charging.

Big consumption items are VHF (24ah/day), Autopilot(40ah/day), Fridge(29ah/day), GPS/Chartplotter(17ah/day), Instruments(21ah/day), running lights(12ah/day),  and miscellaneous items take up the rest.  I have to figure out how to replenish the current used and where to store that current.

A high amp alternator (100A) is the best bang for the buck and I can run the engine for two hours per day to replenish the main batteries.  One 500W Wind generator is dedicated to the refrigerator batteries.  Solar would be nice in theory, but in reality, the sailboat leans, rolls with swells, has limited deck space, and is almost never in the right position to get full sun.  Even in the best conditions, solar would only provide power for 6 hours a day (most of the time less).

Because of the battery location problems I have to deal with, I am adding remote batteries and using the existing low current wiring to charge those batteries.  A 14 awg wire can only handle 10A for a distance up to 12 feet.   12 awg to 16 feet.  10 awg to 28 feet.  I am limiting my current to correspond to the existing wire.

The way I plan to charge them, is to use a VSR that kicks in when the main battery voltage reaches 13.2v (when the alternator is running of if battery charger is on at dockside).  That VSR output is connected to a fuse box.  The low current wires to the remote batteries run off that fuse box.   When the VSR is off, the remote batteries are isolated from each other. 

Example. the VHF uses 24ah a day and a 120ah battery would last only 4 days, unless I add 10ah a day by running the engine/alternator, which would stretch me out to 8 days.  Everything counts on me being able to start my engine.  If the engine won't start, I would reroute the wind generator from fridge over to main batteries and run only critical components of VHF, Lights, and GPS/Chartplotter.

I am still working out the plan, but I may put optional items only under remote batteries and keep essentials on the main batteries.  Optional items are fridge and autopilot (I can steer the sailboat manually if I have to, but 24 hours a day is exhausting).   GPS/chartplotter, VHF, Instruments, fresh water pump, bilge pump, head pump, and running lights are essentials.  The windlass is only used occasionally and while the engine is running.  By offloading the fridge and autopilot to remote batteries, I would only use 100ah a day off main batteries and the alternator would keep that working.  It is possible to get the 70ah a day for optional equipment from the wind generator, and that battery bank could run the fridge and autopilot and supplemented by the current from the VSR.  I could put a remote battery in the nav station, under a battery selector switch, which would be a backup battery to be used in an emergency.  Still analyzing, testing, and deciding where best to put my money to work.

[JW]

[MagnetJuice]

JW,

That picture of the bridge. See that cloud looking thing flying above it? I know that most people think it is a flying saucer. But I know what it is. That is a drone from the Fish and Wildlife Service keeping an eye on you to make sure that the fish that you catch is not under size or an endanger species.

Now, back to the original post. About 40 years ago I read a book. “Getting through to people” by Jesse Nirenberg. It was so good that I re-read it again every couple of years.

By reading that book, I learned to read between the lines and when someone say something, to REALLY get what they are saying.

So I put on my Nirenberg hat and read this entire post again slowly.

I am not going to disclose what I discovered. Instead, I'm going to place myself in the shoes of the OP.

If I were him, (he obviously has money). I would forget about messing around with transistors, diodes and resistors. Instead, I would purchase one 100 AH battery for each of the big consumption items. The VHF, Autopilot, Fridge, GPS/Chartplotter, Instruments and running lights. That is 6 extra batteries, but that's pocket change for me. Then I'll buy another more powerful wind generator to keep all batteries charged.

And now the fun begins. I'll set sail across the great expanse of the Gulf of Mexico and head for the Yucatan. I'll take extra fuel to make sure that I can keep the batteries well charged. Life is GOOD!!! 

When I get to the Yucatan, I'll post on this forum one more time. Not much, just a picture of me soaking the sun on a beach in Cancun and the caption “Wish you were here”. 

A few months later, I'll come back to the States and maybe my resistor from China will be there. Or maybe not. Oh well, when it comes I'll put it in the trash, don't need it anymore.

plasmahunt3r:

If you have a question, state it clearly without beating around the bush. There are some knowledgeable people on this forum that can give you a good answer to solve your problem. Enjoy your boat; weather is getting cooler.

Ed

[JW]

The only other batteries are dedicated to refrigeration, and the wind generator is dedicated to producing cold beer.

I hear that...


That system sounds complex, I think the wind generator is your best bet, since you will get a lot more wind on the water than on land, plus it will make power at night.

MJ you may be right, makes sense because on the side of the bridge that im on is a lobster sanctuary. On the other side you can catch them legally, probably a patrol zone.

On my boat I have florescent tubes in the gunnel's and we would catch shrimp with a dip net under that bridge. I use 12v ballasts to energize the bulbs and there in water proof clear tubes.       

[plasmahunt3r]

My resistors from China still haven't come in, but I found a 1 OHM 100W resistor in my parts box.  So I tested Operahouse's method of just using a resistor and a Schottky Diode.  This works fine.  The current delivered was based on how charged the battery was.  It started at a little over an amp, than tapered down to .49A as the meter hooked on the target battery read 13.6v of charging voltage.

I also redid my transistor circuit setting a voltage trigger of 13.5V.  I am happy with both methods and each has it's place.  The current actually delivered (using either method) to the target battery is based on how charged the target battery is.  As the Target battery reaches fully charged, the current delivered tapers off.  When I add a 1/2 amp load to the target battery while charging, the current delivered increased by that amount.

Attached are the circuits: