SSD relay - or how to charge 2 batteries at one time

Sonnich Jensen wrote in news: snipped-for-privacy@googlegroups.com:

Just use a hard switch. Charging current for the battery cannot be that much so all the wiring and the switch would likely be overkill, but at least it would never die.

Put it on your dashboard. But sure... and SSR would be a drop in replacement that would likely stop your failure modes as well.

Solid State Relay.

In the US we use what's called a 'Battery Isolator' The back to back diodes don't let one battery dishcarge into the other. And allows the alternator to charge both batteries.

Cheers

one for axial stuff, such as light, fridge etc for the living area.

so charge the axial battery.

mediately, the axial battery only when I start moving.

not solve the problem. It happens app every 4 days.

You have some other issue, like with the connecting wires or intermittently failing alternator, that need to addressed. The problem should not persist despite changing relays. Fix that first. As long as all you know is "for s ome reason," you don't know enough. Get to the bottom of it.

There are smarter VSR these days which allow a charger for the aux batt to also charge the starter bit, which sounds like a good idea. Like this:

-or- you can buy a kit if want a hands on project:

With the potential current levels involved in addition to transient hazards on the charging bus, it would be better to stay away from a diy solid stat e project.

the axial battery.

main battery, how can I avoid that?

Furthermore, the diodes allow the most current for the battery with the least voltage...presumably the one that needs charging the most. There could be a problem if you have a HUGE camper battery that's fully discharged and a crap engine battery with high impedance.

While it's easy to fabricate a problematic situation, for most of us, the battery isolator works just fine.

But adding the diode does not allow full voltage to the batteries, especially when i have a solid state alternator. Most of the time i get less than 13V to the batteries, without diodes. I am looking into small voltage booster.

That's news to me. I thought all my vehicles have had sense points at the battery...but I don't have anything newer than 1990.

I assume you've verified that the diode does drop the battery voltage further when inserted at the alternator side of the battery sense point?

, and one for axial stuff, such as light, fridge etc for the living area.

to also charge the axial battery.

ed immediately, the axial battery only when I start moving.

s did not solve the problem. It happens app every 4 days.

harge the axial battery.

al to main battery, how can I avoid that?

cially when i have a solid state alternator. Most of the time i get less t han 13V to the batteries, without diodes. I am looking into small voltage booster.

I just measure the voltage of the batteries at the back of the vehicle, wit hout diodes. When the voltages of the front and back are close enough, the re should not be much IR drops from the wire. But the voltage is not high enough to fully charge the batteries at the back.

I'm confused. If the voltages are the same, and the voltage at the back is insufficient, how can that same voltage at the front be sufficient to charge the front battery?

without diodes. When the voltages of the front and back are close enough, there should not be much IR drops from the wire. But the voltage is not h igh enough to fully charge the batteries at the back.

Yes, i guess it's not charging the front fully either.

e, without diodes. When the voltages of the front and back are close enoug h, there should not be much IR drops from the wire. But the voltage is not high enough to fully charge the batteries at the back.

The OP and I have the same goal of keeping the main(front) battery charged, and drain-down the aux(back) battery if necessary. But most non-relay sol ution produce too much of voltage drop.

I am also thinking about boosting to a higher intermediate battery bank (24 V to 48V) before dropping down to the 12V aux(back) batteries.

You really don't wanna do that.

The assumption is that the circuit that controls the alternator output has a sense line that senses the battery voltage, NOT the alternator output voltage.

You can't disconnect the battery terminal and insert a diode. You have to leave that sense line connected to the battery. In my case, I disconnected the wire at the alternator end and put the diode isolator there. The alternator output went up to compensate for the diode.

You really need to fix the problem with the engine battery. Maybe your alternator is too small and maxed out. Or maybe you're just trying to measure the voltage at idle. If the circuit is working correctly, you'll get whatever the vendor thought the max battery charge voltage should be while driving. The voltage at idle will likely be less. Depending on the loads, the battery may be discharging at idle.

We used to use a schottky diode in the feed to our towed caravan.

We used the same trick to charge my mum's disability scooter when it was in the boot of the car (lead acid gel batteries).

Despite the 0.6V drop, it did the job well enough, was simple and foolproof.

The alternator should be producing around 14V +/-0.5V which leaves adequate headroom for charging.

Whilst the charging is not optimal, it is simple and does give inherent preference to the vehicle's main battery which is a good thing.

Whether it will keep your auxiliary battery topped up with your driving, is something only experimentation will reveal.

Do you mean the relay physically fall out of it's socket? How about a hose clamp or cable ties to hold the relay in place?

George H.

I'm guessing it's 'drop out'.

Silicon Carbide JFET is normally on, take the gate to ground to turn off. And probably add some protection for the gate, just a series R and a zener and small C to stop it straying too far from the source, I'd guess.

UJ3N065025K3S is 25mR, 85A, 650V but sadly quite expensive at USD40. UJ3N065080K3S is half the price but 80mR, 32A, 650V.

Cheers

e:

le, without diodes. When the voltages of the front and back are close enou gh, there should not be much IR drops from the wire. But the voltage is no t high enough to fully charge the batteries at the back.

ack

ont

ged, and drain-down the aux(back) battery if necessary. But most non-relay solution produce too much of voltage drop.

(24V to 48V) before dropping down to the 12V aux(back) batteries.

All the combustion engine vehicles I've seen, the alternator output is tied directly to the battery via a heavy gauge cable and that is the voltage that is monitored and controlled.

And the isolation for charging seems a bit odd. Even at idle, an alternato r is likely putting out enough current to charge, so what if it charges both batteries? The starting battery should be almost fully charged normally an yway. It's seems a nit to insist on only charging it while the RPMs are above a certain level. Usually a battery isolator is used to keep the other battery (which should be a deep cycle type), in reserve so that it can't be run down, eg when using the camper, you don't want to run down the starting battery and wind up stuck.

ht

dle

While that's a true statement, it completely misses the point. The key issue is WHICH END of the wire gets sensed. The link I provided and you snipped shows the situation.

Last time I did this was on a 1975 RV, so current technology may be different.

Again, you missed the point. Design is a tradeoff. The engine speed varies over a wide range, Cost is an issue. The design works for all but the most extreme outlier cases. If you use your clamp-on DC current probe to measure charge current, you might find that the battery isn't charging at idle when all the accessories are turned on. There's enough computing power in current cars to increase the idle speed so you're not discharging. I don't know if they actually do it.

Usually a battery isolator is used to keep the other

The disagreement is not about what. It's about HOW.

ote:

icle, without diodes. When the voltages of the front and back are close en ough, there should not be much IR drops from the wire. But the voltage is not high enough to fully charge the batteries at the back.

back

front

arged, and drain-down the aux(back) battery if necessary. But most non-rel ay solution produce too much of voltage drop.

nk (24V to 48V) before dropping down to the 12V aux(back) batteries.

Given that the cable is ~ 3/8" in diameter between an auto alternator and the battery, run the numbers for 50A flowing through it and you wind up with maybe 30 mv voltage drop for a 10 ft run. And typically the run isn't even that long, so, what's the issue again?

It doesn't show anything about the alleged problem with a typical, large, suitable conductor between the alternator and battery that's in an auto resulting in a voltage drop that requires a separate sensing wire.

In 1975 there was a separate sensing wire at an auto battery?

nator

oth

y anyway.

So what?

I seriously doubt that. It would be a bad design, leaving cars stuck in traffic or similar situations in risk of running down the battery.

That would not be an issue of computing power at all, it's really just measuring and increasing idle speed. Seems a more logical thing is to make sure the alternator has sufficient output at idle so it's not a problem and I'm betting that's the case. People even use inverters to power eqpt during power outages, driven off idling cars, so there must be some reasonable extra capacity there at idle.

Well, if you're isolating it when it doesn't need to be isolated, and isolating it for the wrong reason, making it more complex, that would seem to be an issue to me.

only if stuck there for 10s of hours, engine still going. The main issue with that strategy is it would cycle the battery unnecessarily, killing it earlier.

NT

nd one for axial stuff, such as light, fridge etc for the living area.

also charge the axial battery.

immediately, the axial battery only when I start moving.

id not solve the problem. It happens app every 4 days.

The answer is probably the part about " fridge etc for the living area". LR A is in excess of his alternator capacity and it drops out of regulation ca using the relays to open. The only thing in his system helping to hold the battery bus up is the battery, not the alternator, and the loaded batteries underpowered by the alternator can't make 13.8V under LRA load.

ge the axial battery.

to main battery, how can I avoid that?