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Charging start battery from MPPT


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My MPPT controller is connected to my domestic batteries, but I have a parallel switch as part of the battery isolator cluster between the domestic and start battery isolators. It's for emergencies in case the start battery ever fails.

 

Is there any issue switching the parallel switch on to charge/maintain the start battery from the panels/MPPT? It seems to work fine, just wanted to be sure. I guess there's the potential for the start battery to drain the domestics at night if it's not fully charged or in a worse condition.

 

I realise that I'm essentially making the start battery part of the domestic bank, but I'm not in the habit of taking the domestics below about 75% SoC so I don't think I'll end up draining the start battery too low.

 

I've also got BT batteries at the bow which are linked to the start battery via a relay so switching the parallel switch on will keep them maintained.

 

 

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No battery or electrical problems at all apart from the fact that if a domestic bank cell started shorting you could end up with a flat engine battery. A more elegant and automatic way would be to fit a VSR between the two banks, sensed from the domestic bank (if not bidirectional).

 

However, as engine batteries only ever get discharged by a few Ah per start they are normally all but fully charged so apart from leaving them for months over winter with no charge source so self discharge takes a toll, you may be worrying about not a lot.

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2 hours ago, David Mack said:

Dual sensing VSR. Then both banks get charged by all charging sources, regardless of which bank they are connected to.

 

So that would be between the domestic & start batteries?

 

Would it interfere in any way with the VSR I already have between the start and BT batteries?

 

Right now with the start and domestic banks paralleled all 3 banks are changing.

Edited by blackrose
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31 minutes ago, blackrose said:

 

So that would be between the domestic & start batteries?

 

Would it interfere in any way with the VSR I already have between the start and BT batteries?

 

Right now with the start and domestic banks paralleled all 3 banks are changing.

 

Either VSR will "turn on" when the voltage on the sense side reaches its threshold, so it should not cause problems, but the one with the higher threshold voltage may not turn on until the other bank(s) have charged a bit to allow the charging voltage to rise.

 

With 2 VSRs one would expect all three banks to charge.

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5 hours ago, blackrose said:

My MPPT controller is connected to my domestic batteries, but I have a parallel switch as part of the battery isolator cluster between the domestic and start battery isolators. It's for emergencies in case the start battery ever fails.

 

Is there any issue switching the parallel switch on to charge/maintain the start battery from the panels/MPPT? It seems to work fine, just wanted to be sure. I guess there's the potential for the start battery to drain the domestics at night if it's not fully charged or in a worse condition.

 

I realise that I'm essentially making the start battery part of the domestic bank, but I'm not in the habit of taking the domestics below about 75% SoC so I don't think I'll end up draining the start battery too low.

 

I've also got BT batteries at the bow which are linked to the start battery via a relay so switching the parallel switch on will keep them maintained.

 

 

Exactly what i have, although not quite as nice - just a fused lead with a big crocodile clip on - remove it when running...

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I probably won't bother with the VSR as the start and BT batteries are generally well charged and don't have any draw coming from them while I'm moored. I'll just parallel them occasionally to keep them maintained and make sure I switch the parallel switch off before it gets dark.

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As usual I have a naff, but workable solution. A small cheapo Halfords car battery solar panel. Sits on my hatch and wired directly to the starter battery. So low power that’s fine. I’ve had the setup for years. Never boiled the starter battery and it’s always charged 😊

Edited by Jak
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I have a VSR that connects my domestic batteries to the starter battery.

When solar is on all get charged

When charger is on all get charged

When engine is on both alternators charge both sets of batteries.

When domestics deplete then starter is isolated.

It's not rocket science.

Edited by GUMPY
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24 minutes ago, GUMPY said:

I have a VSR that connects my domestic batteries to the starter battery.

When solar is on all get charged

When charger is on all get charged

When engine is on both alternators charge both sets of batteries.

When domestics deplete then starter is isolated.

It's not rocket science.

this is how mine would be if i left the link in with the engine running but i couldn't determine if it was safe to have both alternators on same circuit?  obviously it is?

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Only proviso is that I set my two alternators* to have exactly the same voltage otherwise one will take control. 😉

 

* Domestic alternator is a variable Prestolite

 

Taking the link in and out when the engine is running and the batteries taking charge results in a 20amp jump in current

Edited by GUMPY
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12 hours ago, David Mack said:

Dual sensing VSR. Then both banks get charged by all charging sources, regardless of which bank they are connected to.

 

I don't really understand why I'd need a dual sensing VSR? The only charging source that doesn't currently charge the start and BT batteries is the solar panels. That's all I'm trying to do. 

 

I don't see why it needs to be more complicated than that?

 

Can anyone recommend a suitable VSR? They all seem to be 140A.

Edited by blackrose
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6 hours ago, Jak said:

Lol. Please refer to my totally workable stater battery / solar suggestion above. Simple, cheap, totally fit for purpose. Just saying?!

 

No, not TOTALLY fit for purpose. It has two potential problems if we ignore the danger of explosion or short circuit when connecting or disconnecting. One is that it involves human nature and one day it will be forgotten, allowing the start battery to be discharged. The second is it will allow a shorting cell in the start battery to flatten both banks. I agree it is cheap and effective.

 

7 hours ago, blackrose said:

I don't really understand why I'd need a dual sensing VSR? The only charging source that doesn't currently charge the start and BT batteries is the solar panels. That's all I'm trying to do. 

 

Neither do I for your specific need.

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46 minutes ago, Tony Brooks said:

 

No, not TOTALLY fit for purpose. It has two potential problems if we ignore the danger of explosion or short circuit when connecting or disconnecting. One is that it involves human nature and one day it will be forgotten, allowing the start battery to be discharged. The second is it will allow a shorting cell in the start battery to flatten both banks. 

 

Lol 🤣

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14 hours ago, Jak said:

As usual I have a naff, but workable solution. A small cheapo Halfords car battery solar panel. Sits on my hatch and wired directly to the starter battery. So low power that’s fine. I’ve had the setup for years. Never boiled the starter battery and it’s always charged 😊

I have used a similar method in the past. Such a small panel it didn't even have a solar regulator. One-two-both switch, which is open to human error, but it becomes habit to switch it when turning the engine off. 

 

Sometimes the simplest method is the best 

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37 minutes ago, rusty69 said:

 

Sometimes the simplest method is the best 

 

Given what Tony says about the disadvantages of directly connecting 2 banks, the simplest method for me is to put a VSR between the start and domestics. I certainly wouldn't want an additional little panel charging the start battery which doesn't sound like it's even regulated by a controller. I've got 910w of panels and MPPT so there's no need for more panels & wiring.

Edited by blackrose
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1 minute ago, blackrose said:

 

Given what Tony says about the disadvantages of directly connecting 2 banks, the simplest method for me is to put a VSR between the start and domestics. I certainly wouldn't want an additional little panel charging the start battery which doesn't sound like it's even regulated by a controller. I've got 910w of panels and MPPT so there's no need for more panels & wiring.

Sure. Whatever works for you. Anything you add has the possibility of failure though. 

 

I eventually bought a 10 quid dual battery pwm regulator that charges both the starter and domestic batteries. 

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8 hours ago, blackrose said:

 

I don't really understand why I'd need a dual sensing VSR?

You don't, if all the charging sources are connected to the same (domestic) bank. But you do if there are charging sources connected to both banks.

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5 minutes ago, David Mack said:

You don't, if all the charging sources are connected to the same (domestic) bank. But you do if there are charging sources connected to both banks.

 

And you want all charging sources to charge all banks.

14 minutes ago, blackrose said:

I certainly wouldn't want an additional little panel charging the start battery which doesn't sound like it's even regulated by a controller.

 

I can't remember the relative sizes, but a small panel charging a large battery or bank does not really need a controller. In the early days of solar using a controller to keep batteries topped up was rare, and the panels were all small, only a few watts. I have a feeling that a panel of 10% or less of bank capacity does not need a controller, but please check those numbers.

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37 minutes ago, Tony Brooks said:

 

And you want all charging sources to charge all banks.

 

I can't remember the relative sizes, but a small panel charging a large battery or bank does not really need a controller. In the early days of solar using a controller to keep batteries topped up was rare, and the panels were all small, only a few watts. I have a feeling that a panel of 10% or less of bank capacity does not need a controller, but please check those numbers.

That is certainly the principle I was working on at the time, though can't remember the exact percentage. Perhaps @Jakuses the same idea, though pwm charge controllers are so cheap these days, it's little extra cost to add one. As I said above I have a cheap dual battery pwm one that you can set different percentage charges on for domestic and start battery. 

IMG_20230528_094354924.jpg

Edited by rusty69
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Well that told me then!  The Halfords panels are tiny things. Just designed to keep a car battery topped up if you don’t use it much. No controller needed. I’ve had it a few years and it’s doing a good job so far. 

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1 minute ago, Jak said:

Well that told me then!  The Halfords panels are tiny things. Just designed to keep a car battery topped up if you don’t use it much. No controller needed. I’ve had it a few years and it’s doing a good job so far. 

I got the impression at least two of us were agreeing that it was a workable solution. Maybe I read it wrong. 

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42 minutes ago, rusty69 said:

I got the impression at least two of us were agreeing that it was a workable solution. Maybe I read it wrong. 

 

 got Jak mixed up with whoever is using a jump lead. I agree that Jak's solution is simple enough and has no significant downside, however with the modern large solar arrays I think the VSR is a more elegant solution because it, in effect, it uses charge that would otherwise be wasted.

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