Split Charge Diode - Over Charge Risk

[SimonP85]

We have a single output mains battery charger (8A) connected to both starter and domestic batteries via a diode splitter. The sense wire from the charger is on the domestic battery.

The starter battery is 90Ah and the domestic 160Ah. How concerned should I be about the possibility of over charging the starter battery is the domestic is far more depleted?

I'm aware there are better options than a diode splitter (e.g. VSR) but if this arrangement is acceptable then I see no need to alter it. I just want to ensure both batteries are well looked after.

Many thanks.

 

[pvb]

You won't overcharge the start battery - in just the same way as your car's alternator doesn't overcharge the car's battery.

As you have a sense wire attached, a VSR would perform in just the same way as the diode splitter.

 

[SimonP85]

Pvb,

Many thanks for the quick response. Just to complete my understanding - I assume the starter battery will take current from the diode splitter to being with, but as its voltage increases the current will favour the domestic bank (assuming the voltage here is still lower) and over time almost all current will be flowing only to the domestic side?

 

[pvb]

Pvb,

Many thanks for the quick response. Just to complete my understanding - I assume the starter battery will take current from the diode splitter to being with, but as its voltage increases the current will favour the domestic bank (assuming the voltage here is still lower) and over time almost all current will be flowing only to the domestic side?

Yes, batteries accept current according to their needs.

 

[fergie_mac66]

Would a split charge diode work with solar panels.

 

[xeitosaphil]

Would a split charge diode work with solar panels.

I have a Bi Directional vsr fitted with a solar panel and engine charging, and the solar panel keeps engine and domestic battery banks at 100% with enough sun?

 

[VicS]

Would a split charge diode work with solar panels.

I have a Bi Directional vsr fitted with a solar panel and engine charging, and the solar panel keeps engine and domestic battery banks at 100% with enough sun?

yes but you will lose approx 0.7 volt across the diodes.

The OP's charger apparently has a battery sensing connection which enables it to compensate for this voltage loss.

A VSR does not suffer from such a voltage loss. A bidirectional one can be wired so that the starter battery is the first priority for the alternator while the domestic battery can be first priority for other charging systems

 

[SimonP85]

Thanks all. Good to know the current setup is suitable.

 

[VicS]

Thanks all. Good to know the current setup is suitable.

My only very slight concern would be that there will be a sightly higher voltage drop across the diode supplying the charge current to the domestic battery than there will be across the diode supplying supplying very little current to the fully charged starter battery. The former being compensated for by the battery sensing of the charger,

Simple remedy would be to fit a switch between the diode splitter and the starter battery if you find it needs the electrolyte topping up frequently. The starter battery need only be connected if it requires charging , which ordinarily it should not.

 

[fergie_mac66]

I have a Bi Directional vsr fitted with a solar panel and engine charging, and the solar panel keeps engine and domestic battery banks at 100% with enough sun?

yes but you will lose approx 0.7 volt across the diodes.

The OP's charger apparently has a battery sensing connection which enables it to compensate for this voltage loss.

A VSR does not suffer from such a voltage loss. A bidirectional one can be wired so that the starter battery is the first priority for the alternator while the domestic battery can be first priority for other charging systems

TThank you

 

[SimonP85]

My only very slight concern would be that there will be a sightly higher voltage drop across the diode supplying the charge current to the domestic battery than there will be across the diode supplying supplying very little current to the fully charged starter battery. The former being compensated for by the battery sensing of the charger,

Simple remedy would be to fit a switch between the diode splitter and the starter battery if you find it needs the electrolyte topping up frequently. The starter battery need only be connected if it requires charging , which ordinarily it should not.

Thanks VicS.

The starter battery is of the sealed variety so nothing to top up.

The only other thing I have just recalled is we have solar panels connected directly to the domestic battery, but I assume this won't interfere and the battery will still just take the charge/current it needs.

 

[VicS]

Thanks VicS.

The starter battery is of the sealed variety so nothing to top up.

The only other thing I have just recalled is we have solar panels connected directly to the domestic battery, but I assume this won't interfere and the battery will still just take the charge/current it needs.

With a sealed battery I think I would fit a switch to disconnect it from the charger and only charge it if and when necessary ...... or a brief top up now and then if the engine has not been run.

Solar panel should cause no problems

Your system begins to sound as though a bi-directionla VSR rather than diode splitter might be a better set up. Starter battery first priority for the alternator. Domestic battery bank first priority for the charger and solar.
A switch in the VSR's negative connection could be used to prevent it closing and charging the starter when that's not necessary

 

[BabaYaga]

I agree with VicS that there is a slight risk of over charging the starter battery due to the difference in voltage drop over the two diodes (which relate on the current flow). This difference will typically be at maximum when the starter battery is already fully charged while the domestic battery is heavily depleted – but at that point the output voltage from the charger will likely be quite low, so that the compensation for voltage drop provided by the sense lead would probably not overshoot say 14,4V at the starter battery.
As the domestic charges up and the voltage rises though, I think there is a certain risk for such a situation to develop. I would not put a sealed battery to that risk, so I agree a switch might be a good idea.
Also, I believe over charging is an issue on two time scales. Apart from over shooting 14,4V for too many hours there is also sitting at 14,4V for too many days. Some chargers might not drop down to float voltage if there is a parallel load on the system.

 

[xeitosaphil]

TThank you

AS Vic says , a VSR doesn't suffer voltage loss.
The Bi directional VSR has the option to charge which ever battery you decide to choose to sense as priority, engine or domestic.
It has a cut in and cut out Min and Max voltage, so when the sensed battery reaches a pre set voltage it automatically joins the second battery for charging to charge both batteries to a max of 14.4v then it disconnects both. Once the sensed battery reaches a min voltage it disconnects second battery so charging continues until to dual connection voltage is reached again charging both again. All fully automatic.
I have my sensing wired to the domestic set along with the alternator feed wire as this is my biggest depleted battery bank most times which needs charging first. Engine start takes so little out of the battery it takes no time to charge the loss.
I can however start from both battery banks as a precaution if the dedicated engine battery becomes flat.
100w Solar panel keeps both banks charged automatically and in best state of charge as long as there is enough sun.

Should also state, I have a switch in the sensing wire feed so I can disconnect ability to charge engine battery, should the domestic battery require high charge rate due to possible 50% discharge.

 

[pvb]

As the domestic charges up and the voltage rises though, I think there is a certain risk for such a situation to develop. I would not put a sealed battery to that risk, so I agree a switch might be a good idea.
Also, I believe over charging is an issue on two time scales. Apart from over shooting 14,4V for too many hours there is also sitting at 14,4V for too many days. Some chargers might not drop down to float voltage if there is a parallel load on the system.

Maintenance-free batteries are fine with 14.4v for long periods, the OP's starter battery isn't going to be damaged. The Adverc booster on my old boat used to cycle the maintenance-free batteries between 14.4 and 14.8v, it didn't harm the batteries, unless you reckon 8 or 9 years isn't a reasonable life for domestic batteries.

As we don't know what charger he has, speculating on whether it might or not not drop to a float voltage seems pointless.

 

[pvb]

The Bi directional VSR has the option to charge which ever battery you decide to choose to sense as priority, engine or domestic.
It has a cut in and cut out Min and Max voltage, so when the sensed battery reaches a pre set voltage it automatically joins the second battery for charging to charge both batteries to a max of 14.4v then it disconnects both. Once the sensed battery reaches a min voltage it disconnects second battery so charging continues until to dual connection voltage is reached again charging both again.

I don't recognise that description of a VSR, which VSR do you have?

 

[xeitosaphil]

I don't recognise that description of a VSR, which VSR do you have?

I have a Merlin VSR http://www.merlinequipment.com/markets/group.asp?groupid=18

http://www.jgtech.com/pdf/VSR Manual.pdf

 

[SimonP85]

Thanks all for the replies.

To be honest as long as it's not going to explode I'm not too concerned. If it's slightly less ideal and reduces the life of the battery by a year then that's not too much of a concern. Just nice to know we can potentially leave the charger on over night or similar to ensure the batteries are both fully charged and maintained fairly well.

Cheers

 

[lw395]

You won't overcharge the start battery - in just the same way as your car's alternator doesn't overcharge the car's battery.
....

Many car batteries curl their toes up at between 50 and 100k miles.
That is probably something like 1000 to 2,500 hours at 14+ Volts.
That's about 40 to 100 days of being on the mains continuously.
That's an extreme way of looking at it, but it's quite possible to be reducing the life of the engine battery excessively.
It will depend on how the house batteries get used and charged.

But the comparison with a car is at best, not helpful.

 

[VicS]

Thanks all for the replies.

To be honest as long as it's not going to explode I'm not too concerned. If it's slightly less ideal and reduces the life of the battery by a year then that's not too much of a concern. Just nice to know we can potentially leave the charger on over night or similar to ensure the batteries are both fully charged and maintained fairly well.

Cheers

Overnight ! No worries. Concern was based on the possibility that you left the charger on for a week or several at a time.

However in the normal course of events the starter battery should not need this overnight charging. You don't put your car battery on charge every night do you? ....... and if its a modern car there will be a small ( a few mA or so) drain all the time that your boat engine start battery does not have to support.