looking for new ideas, voltage sensing relay/charge splitter

[JeanF]

EDIT : I’ve simplified my prose a little bit, sorry for the wall of text that was here before. The post is still long but hopefully a bit easier to read.

TL;DR : what do you suggest to charge 2 lead-acid batteries (first, a small one, then a bigger one) from a wimpy 6 amp alternator, without wasting too much current in a big relay coil (such as those inside readily available VSRs) and preferably using off-the-shelf components?

Hi,

I'd like to charge two batteries from one single small outboard motor on a small sailboat...

- The engine is a small outboard (9.9 HP) there is a single cable (2 conductors) between engine and battery; no separate alternator and starter cables.
- the starting battery is a small (20 Ah) 12V lead-acid
- the alternator/dynamo is rated 6 A, which is much smaller than “proper” alternators on bigger engines/boats
- the secondary battery is lead-acid too, about 60 Ah IIRC, but will probably be upgraded in the future

Yes, I know that a 6 A current source is not enough to be the only power source on the boat if power-hungry devices were to be installed. Other sources such as solar panels would make sense... For now there are only a few LED lights and 6 A is enough. And, even with solar panels installed (not planned for now), I’d still be interested in putting this 6 A current to good use rather than wasting it by boiling the small starter battery when the engine runs.

The obvious answer is to just buy a voltage sensing relay and forget about it, but:
- commonly available VSRs/ACRs/whatever are rated for huge currents, around 100 or 140 A (massively overkill here)
- the coil current of these relays is substantial, around 300 mA; that's already 5% of the available current wasted
- the quiescent current of these is a bit large for my taste (around 10 mA). Not the end of the world, but I'm sure it’s possible to do better
- these VSR just put the two batteries in parallel when the first one gets to a specific voltage threshold. Charge from the starter battery is dumped into the secondary battery, but this is only a short current peak and the efficiency of the transfer is poor. That wouldn’t matter in a more traditional use case with a big alternator, because the alternator would then charge both batteries, thus quickly compensating the loss. Here I’d rather charge one battery, then the other, without putting them in parallel.

Until now, I've been using  a Victron diode-based “battery isolator” (just two diodes back to back), plus a 30A relay across one of the diodes to allow reverse current when the starter motor engages.

This required a small mod on the stock wiring loom of the outboard, which I'd like to avoid, because non-standard wiring is a pain when somebody else has to work on the motor.

Also, there’s a flaw with the “2-diode” charge splitter; the current will flow to the most discharged battery. If there is a shorted element in the secondary battery, or a leakage to ground somewhere, that could prevent the starter battery from charging properly.

I’ve been thinking about removing the diodes and using a SPDT relay of appropriate size (the starter circuit is fused by a 20 A automotive fuse inside the motor, so I guess the starter motor current is in that range) with Batt 1 on the NC contact, Batt 2 on the NO, and the motor on the common contact.

In this case the motor would be connected by default to the starter battery, and I'd need a small circuit to detect when it's fully charged and energize the relay to send the charging current to the other battery. With this, the batteries are always independent.

I could tear down an existing VSR and replace the big SPST relay with a smaller SPDT one. But they are potted so that may be a difficult task. Besides, even cheap ones are not that cheap; and doing this would do nothing to improve idle current. All in all, bad idea.

So, if I want to roll my own voltage sensing relay, I need a board to sense voltage, while drawing the smallest possible quiescent current when off. I could design my own (TL431 + comparator, or a small µcontroller…) but before reinventing the wheel I’d like to check if something off-the-shelf exists, that would save some development time.

There are many variations of little boards on ebay for "battery protection" with a relay, some with a LM358, some with a LM393, some a 555 (!) and trimpots... Don't know if there is a voltage reference on board though, and I'm a bit hesitant to use these boards in a system which must be "reliable"...

A commercially available voltage sensing relay board with a (not too big) SPDT relay and a voltage reference, and not too much idle current would be ideal, but I couldn't find one.

How would you do it? Do you know of off-the-shelf little modules that I could use to detect when Battery number 1 is charged and drive a relay accordingly?

I’m probably overthinking this with voltage detection and a relay.. Please feel free to tell if you have another (possibly simpler) approach

Thanks for any ideas

[Renate]

Oh, my situation is totally different from you, my house doesn't float.

I think that you have to come up with something better to keep your house battery charged.
Just tacking it on to a grossly underpowered alternator is not much of a solution.

The amount of current out of that is in the realm of "keep alive".
I have a 2800 W Onan generator and it has windings for 12 V.
I have no idea what the capacity is, but it's basically enough so that the generator doesn't wear down the battery.
When the AC generator is running it usually power my 60 A charger.

I don't like having relays around the starter current.
I'd either just leave it alone, straight to the battery or break the starter out straight to the battery and make the tiny alternator separate.

Of course, all of this is influenced by how much you run the engine and how much house current you use.

Me?
80 AH truck battery (type 78), 100 A alternator, 454 V8
224 AH house battery (2 AGM GC2), 600 W solar, 60 A AC charger, (seldom used).
2800 W AC generator (seldom used).
Emergency tie relay between the two batteries.

[JeanF]

Hello again and apologies for being late; thank you for your reply,

I’ve edited my OP to make it clearer, it’s still long but I hope that helps a little bit.

Yes I know that little alternator isn’t going to provide megawatt-hours of energy, but it’s better than nothing. For now there are only a few led lights onboard, so current draw is minimal.

I see where you’re coming from about relays and starter current, but the piston displacement of your engine is about 35 times greater than mine so indeed the starter current in your setup must be hundreds of amps, and appropriately sized relays would be impractical. Here I’ve not measured the starter current, but the starter wiring is protected by a 20 A fuse under the hood. Any automotive relay would probably be enough…

Making the alternator separate is not an option here; the whole point of this is not only to charge batteries (the previous solution with 2 diodes worked well enough) but also to put the motor back to its original, un-modified state.

I’ve looked at "big" VSRs for boat or RV use, Victron Cyrix CT, Kemo M148A (originally a low-batt protector but can be adjusted ; so it could be used to toggle a relay around 14 volts), Beaufort Electronics V-Guard II, as well as a range of battery couplers from local RV-equipment sellers (namely Scheiber and Inovtech), and compared their current ratings and idle currents.

I think I’ve found a nice product, the SBS range of “battery protectors” from bed-electronics in Latvia. The quiescent current is only 0.15 mA, it’s possible to set both “connect” and “disconnect” thresholds, their form factor is small and they look well made. Their use is not often discussed on forums but they seem to have a good reputation. I think I would trust these a lot more than a random module from Aliexpress.

They are, like the Kemo M148A, sold as low battery protectors, and the voltage range of the 12V model is obviously too low to be used as a “full battery monitor”, but the manufacturer also makes versions of these for lithium batteries, so 4S version could be used (range: 12.5 to 15.5 V) to drive a relay.

Here is what I have in mind:


when the starter battery reaches some threshold (to be determined, probably around 14 to 14.5 V), the voltage sensor activates the relay. The relay stays latched thanks to D2.

D2 could be replaced by a short, but I figured a diode would be better, because the product I intend to use as a voltage monitor (see above) has a P-type MOSFET between input and output, so D2 is preventing current from flowing from Batt 1 to Batt 2 through the “voltage sensor” PCB, for a few ms, before the relay had enough time to close.

D1 is necessary to allow the relay to switch back to the starter battery when the engine stops. I could use one of the big Schottky diodes that are inside the Victron “isolator” that I already have, but the voltage drop is a bummer. I wonder if one of the ready-made ideal diode modules from Aliexpress could be suitable here…

What do you think?

Another possibility would be to keep B1 permanently connected to the motor and use the relay to put B2 in parallel when the threshold is reached. That’s what commercial VSRs do. But I’m afraid that if B2 is empty, doing that would completely discharge B1 (which is smaller) and the motor would then take ages to charge both at once. And if we stop the motor because the wind comes back, we then have a starter battery which is less charged than before.

With the circuit above, the two batteries would stay independent, but I must admit I’ve not seen anyone doing that sort of thing on the Internet…

Thanks again everyone, I’d be glad to learn from other peoples experience

[Renate]

Yeah, that is a head scratcher there.
I understand that you want the outboard to stay a stock two conductor.
Obviously you want to avoid D1.

In other circumstances you might use the "S" (stator) connection out of the alternator to see if the engine is running (to know when to switch back).
Or even just the oil light.
You could put some sort of current sensor in.
The relay doesn't have to switch back to B1 very quickly, a few seconds wouldn't hurt.
How about using a microphone and a sound detector?

[JeanF]

Yes, that’s a fun little puzzle

Avoiding D1 could be good but I have to be careful not to introduce too much complexity, or too many failure points, just for the sake of removing D1... After all, with the previous system, there were voltage drops across the diodes, too. And that seemed to be OK, every time I was aboard I checked the voltages and they looked fine. I don’t really know if the batteries are in good shape, I’d have to measure their capacity for that, and I haven’t been able to see my parents/the boat yet, due to Covid.

I suspect the voltage regulation of the motor’s output is very minimalist, if even existent. With the old motor that was there before (it didn’t have the electric start option back then, but was also used to charge the battery), the voltage went sky high and cooked a few batteries. Back then I “solved” the problem with a small solar regulator (without MPPT) between the motor and the battery.

So, this time I guess (but again, without really knowing) that I can just monitor the voltage and assume that “high enough voltage == battery charged”, I may be wrong.

I got absorbed too by the question “how can I tell if the engine is running without tapping into its harness”

I’m not sure a microphone based system would be reliable enough without gluing the mic to the engine, which, again, is a mod that I’d rather avoid. Current measurement could indeed be a good option.

I wanted to use one of these cheap ACS712 (or another Allegro sensor variant) boards that are all over ebay. That could be neat because their output is centered on 2.5 V, so with a TL431 and a comparator I could make a fully analog solution, which means, no code proofing, less risk of lockup due to transients, etc. But these sensors draw 10mA, that’s too much to be kept on permanently.

There are nice integrated high side current monitors that could be directly powered from 12 V or more, and have low Iq, made by TI or LT, but then I’d have to design a custom circuit, add a shunt, make a PCB, make all that reasonably rugged, etc. This is for me, as a beginner, really a lot of work; and I am not sure I’m ready to go through all this trouble just to avoid a 0.5 V drop.

Besides, a “engine running” detector isn’t enough; a voltage sense would still be needed to decide when to switch between batteries. So I gave up and thought that maybe a solution based on voltage measurement only could be sufficient...

But I’m not against suggestions on this topic, on the contrary!

I wonder whether there is something on the circuit diagram above that I may have overlooked, and that will come to bite me in the back later, or if it’s reasonably OK as is...

[Renate]

I don't know outboard engines, particularly at that size.
Does it actually have a rotor field coil and a regulator?
Or is it just some permanent magnet rotor?

[JeanF]

I don't know for sure. I think it may very well be a permanent magnet on the flywheel, plus a simple coil on the stator. On outboards of this size there are often little aluminium boxes with fins that are called "rectifiers" or "rectifier-regulators" in the service manuals (when you can get them). I don't know outboard motors very well either, I don't know what's inside these boxes. Probably just a couple of diodes + maybe a shunt regulator in the ones that claim to be regulated.

There may be variations between brands/models though. In this case it's a Mercury ProKicker 9.9HP from 2013 or 2014 I think.