Battery Charging Diode

Tim Good

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I have three banks of batteries. But when the engine is on, only the house bank charges.

It’s been like this for a while and it’s been manageable but I’m looming to improve.

Below is a diagram I got from the original owner. It shows a diode with the alternator going to the third prong.

However in reality the alternator goes to the house bank prong. See photo.

I have a couple of questions.

What is this diode meant to do? Share the charge amongst all banks?

At some point the previous owner moved the alternator feed from the middle prong to the house bank prong. Can’t think why but presumably I need to move it back to the middle prong?
 

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pvb

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The blocking diode allows the alternator to charge several separate batteries (or banks) whilst keeping them electrically isolated so that they can't discharge into another battery.

The input to the blocking diode is the post marked in red - this is where the alternator should be connected.

If it doesn't work when you change the connection, it could be that the diode has failed and needs replacing. If you replace it, there are now much better low-loss isolators available (eg Victron ArgoFet).
 

halcyon

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First job is to check voltage on terminals with engine running, then decide how much money you want to throw at it.

Brian
 

Stemar

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The only logical reason I can see for doing that would be because it wasn't working properly - one of those "temporary" bodges to get you through this cruise.

I'd be inclined to replace it with a Voltage sensitive relay. Feed the alternator output direct to the starter battery, with the VSR feeding the house bank. The bow battery could have a 12v-12v charger system or its own VSR.

Actually, I have a cheap as chips car headlamp relay fed from the field terminal on the alternator, and it's worked for years, but that's because VSRs were a lot more expensive then, and I was feeling poor but, if I did it now, I'd use a VSR
 

TernVI

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Do you have solar?
Any VSR's?
Shore power?

It's best to consider the system as a whole and be really clear what you want it to do.
Also that 'diode' is obviously more than just a diode.

Some people have used diodes in various ways. You can use a diode splitter to charge batteries equally.
If you connect the alternator to the house bank directly, and a diode from the house to the engine battery, the engine battery gets a lower level of charge due to the diode drop, which might approximate to what it actually needs. I recall boats wired like this at the turn of the century...
Give the house batteries a boost charge at 14.5V and the engine battery gets about 13.8 or 14V which is plenty to keep it topped up. (Depending on use patterns) Not saying I advocate this, just explaining the concept.
 

david_bagshaw

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The argofet is much better but check it will fit in your space as it has a larger footprint as well.

the diode block has in all likely hood failed as they do get warm especially on the alternator in connector.

voltage drop across a good diode block .5 to.8 volts argofet .05 volts. voltage sensitive relay 0 volts
 

TernVI

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It's just being used as 3 studs to join cables together I suspect.
If it's no longer needed, consider getting rid of it.
 

Tim Good

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Ok thanks all. I’ll reconnect the red terminal to the alternator and see what happens. I have a good battery monitor to see what each bank is up to.
 

RobbieW

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Ok thanks all. I’ll reconnect the red terminal to the alternator and see what happens. I have a good battery monitor to see what each bank is up to.
You might want to consider whether the alternator has been upgraded and is the splitter specced for the upgrade. I had that where the engine had been changed with a higher output alternator but the splitter was original. It got very hot and was damaging cable insulation. Short term I did what you see, then fixed it properly.
 

Tim Good

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Feedback:

ok so I’ve put the alternator on the red post and nothing is getting charged. So presumably the diode went caput at some point and they moved it to the house bank.

I’ll look to replace.

In the mean time there’s nothing to stop me putting the alternator lead to the engine post presumably? My house bank has 500w of solar filling it up so doesn’t really need help from the alternator.

Likewise, presumably I could connect my solar to the Red post and charge everything? Or would that confuse the solar controller?
 

PaulRainbow

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Feedback:

ok so I’ve put the alternator on the red post and nothing is getting charged. So presumably the diode went caput at some point and they moved it to the house bank.

I’ll look to replace.

Look at the Victron ArgfoFET, mentioned earlier, good kit, less volt drop.

In the mean time there’s nothing to stop me putting the alternator lead to the engine post presumably? My house bank has 500w of solar filling it up so doesn’t really need help from the alternator.

Nothing at all, you can connect to any output you choose.

As TernVI suggested earlier, now might be a good time to consider an overall charging strategy, before spending any money. Consider each charging source and how it will charge all of the banks that it needs to charge.

A 3 output ArgoFET will allow the alternator to charge all three banks.
Do you need to charge all 3 banks by solar (especially the engine battery) ?
How does the bow battery get charged ?
What doe the mains charger charge ?

Likewise, presumably I could connect my solar to the Red post and charge everything? Or would that confuse the solar controller?

That's generally not going to work, the solar controller won't "see" the battery voltage through the "diode". Consider other options, see above questions.
 

TernVI

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Feedback:

ok so I’ve put the alternator on the red post and nothing is getting charged. So presumably the diode went caput at some point and they moved it to the house bank.

I’ll look to replace.

In the mean time there’s nothing to stop me putting the alternator lead to the engine post presumably? My house bank has 500w of solar filling it up so doesn’t really need help from the alternator.

Likewise, presumably I could connect my solar to the Red post and charge everything? Or would that confuse the solar controller?
You've owned this boat for a while?
Any of the batteries gone flat in this time?
How are your batteries getting charged?
What are you trying to achieve exactly?
 

Tim Good

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1. You've owned this boat for a while?
2. Any of the batteries gone flat in this time?
3. How are your batteries getting charged?
4. What are you trying to achieve exactly?

1. yes. The solar charges the 4 house batteries. Every couple of weeks I bring the batteries together and the others get charged. Not ideal I know but it’s worked for a number of years.

2. No the 4 trojan house batteries are topped up regularly and I check the specific gravity every 2/4 months and top up as required. The bow and engine batteries I recently had tested and are ok in capacity. About 75 percent of new.

3. solar or main bank, mains charger when hooked up and alternator but just for house at moment. As above I bring everything together occasionally to charge engine and bow.

4. Making better use of the alternator to keep bow and house topped up and not having to faff bringing them all together to do that.
 

PaulRainbow

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4. Making better use of the alternator to keep bow and house topped up and not having to faff bringing them all together to do that.

How about:

Leave the solar connected to the domestic bank.
Connect the alternator to a Victron ArgoFET with two outputs, one to the engine battery, one to the domestic bank.
Fit a Victron VSR between the domestic and bow banks.*
Mains charger connected to all three banks, if it's a 3 output model, or just the domestic if only a single output.

*Depending on what the bow battery is doing and the cabling to it, you may need an additional relay to stop it blowing VSR fuses under load.
 

TernVI

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So what you seem to want is something to put a fairly small amount of charge into the bow and engine batteries?
The solar/alternator/house bit meets your needs?

I don't think a diode splitter is what I would look at these days. I would think about leaving the house/alt/solar exactly as it is and using a VSR or B2B system to charge the other two batteries.
Or a small solar regulator for each, if you were happy for them not to get charge from the alternator.
Whatever you do, I don't think you will completely eliminate the need to keep an eye on charge levels, but you could reduce the amount of intervention needed. Sometimes the need to intervene regularly is actually a good thing, it means systems get cheecked and problems are found in good time.
 

PaulRainbow

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So what you seem to want is something to put a fairly small amount of charge into the bow and engine batteries?
The solar/alternator/house bit meets your needs?

I don't think a diode splitter is what I would look at these days. I would think about leaving the house/alt/solar exactly as it is and using a VSR or B2B system to charge the other two batteries.
Or a small solar regulator for each, if you were happy for them not to get charge from the alternator.
Whatever you do, I don't think you will completely eliminate the need to keep an eye on charge levels, but you could reduce the amount of intervention needed. Sometimes the need to intervene regularly is actually a good thing, it means systems get cheecked and problems are found in good time.

Connecting the engine battery to a bank of Trojans, being charged at 14.8v from a 500w solar array will make the engine battery very short lived.

Come to that, the bow battery might not be too happy.

Depending on the batteries being used, a B2B might be a better choice for the bow battery and leave the engine off of solar charging.
 

Tim Good

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What make/type of batteries are the engine and bow batteries ?

Engine:
Bosch T4 800a / 140ah. Fully overnight drop tested recently at 58% capacity but the graph shows it’s cranking amps tailing off quite quickly with voltage. Approx 9 years old

Bow:
Optima Yellowtop 975a / 75ah. Fully overnight drop tested recently at 78% capacity. Approx 9 years old.

Another thing to note is that I do have a small 60w panel on the coach roof. It also goes to the house bank. I could leave my main 500w solar bank on the house. Attach the alternator to the bow-battery and then the small solar panel to the engine battery directly. Or small panel to engine and alternator to engine.
 
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