PabloPicasso
Well-Known Member
Where can i buy a diode? I;ve read this is what is needed to add a second battery so I can have a dedicated start battery and a domestic one.
diodes
- Thread starter PabloPicasso
- Start date
prv
Well-Known Member
The whole area is a bit of a can of worms, but I think most people nowadays would agree that you don't want a diode, you want a voltage-sensitive relay.
What this does is connect your two batteries together while the engine is running, so that they both get charged. When the engine stops, the relay disconnects them so that only the domestic battery gets run down, leaving the engine battery full for the next start.
A diode does a similar job, although instead of connecting and disconnecting it acts as a one-way valve. The problem with a diode is that it takes up some of the voltage across itself, so that the batteries don't charge as effectively.
Pete
What this does is connect your two batteries together while the engine is running, so that they both get charged. When the engine stops, the relay disconnects them so that only the domestic battery gets run down, leaving the engine battery full for the next start.
A diode does a similar job, although instead of connecting and disconnecting it acts as a one-way valve. The problem with a diode is that it takes up some of the voltage across itself, so that the batteries don't charge as effectively.
Pete
DavidGrieves
Well-Known Member
Hi
Have a look here
http://www.tb-training.co.uk/MarineE09.html
If you are going to fit a 1 2 both type switch you could just take the alternator charge wire to the common side of the 1 2 both switch. But now you have to manage the charging of each battery your self. Less volt drop this way, but might be easier for you if you are not into elecs.
Split charge diode is a fit and forget device but as already said has a voltage drop across it, but has advantage of not being able to go open circuit from alt to battery.
This has got to be the best system (I think so anyway. stand by to be shot down...!)
http://www.sterling-power.com/products-altbatt.htm
But £250
Have a look here
http://www.tb-training.co.uk/MarineE09.html
If you are going to fit a 1 2 both type switch you could just take the alternator charge wire to the common side of the 1 2 both switch. But now you have to manage the charging of each battery your self. Less volt drop this way, but might be easier for you if you are not into elecs.
Split charge diode is a fit and forget device but as already said has a voltage drop across it, but has advantage of not being able to go open circuit from alt to battery.
This has got to be the best system (I think so anyway. stand by to be shot down...!)
http://www.sterling-power.com/products-altbatt.htm
But £250
pvb
Well-Known Member
Err, no. Diodes can fail in "open circuit" mode.
halcyon
Well-Known Member
Diodes can fail short circuit or open circuit, plus depending on type can drop 0.5 to 1 volt. So you reduce recharge levels dramatically, or you need to spend more money on a alternator regulater.
Unless you have a mega alternator, you can fit a cheap VSR, for example Durite on a thread the other week. They offer no volt drop, alternator always connected to engine battery, so any fault in split charge system does not loose charge to engine battery.
Brian
mikefleetwood
Well-Known Member
The simple answer to the OP's question "where can I buy a diode" is RS, or similar, however, he would need to know what kind of diode he needs.
I would suggest (and I'm not being deliberately harsh) that OP probably needs to discuss what he wants to do with a suitably experienced marine electrician - or at least someone who has already done the same as he is considering.
I would suggest (and I'm not being deliberately harsh) that OP probably needs to discuss what he wants to do with a suitably experienced marine electrician - or at least someone who has already done the same as he is considering.
halcyon
Well-Known Member
The OP question was :-
I;ve read this is what is needed to add a second battery so I can have a dedicated start battery and a domestic one.
I think most answers relate to fitting a dedicated start battery and a domestic one.
Or have I read something wrong.
Brian
William_H
Well-Known Member
Diodes
The concept is to provide a separated battery for domestic lights etc which is isolated from the battery used for engine starting. So that if you use lights too much you still have a battery to strt the engine to recharge batteries.
You can isolate the batteries and circuits very easily. The problem is that you must connect the batteries together to charge them when the engine is running (having only one alternator) and of course you must disconnect (isolate) the service battery when the engine is stopped. You can do this with switching (1,2,both switch) but you must remember to switch to both when engine is running then separate the batteries again when engine is stopped.
Now a VSR is a voltage sensitive relay which senses when the engine battery rises (which means it is on charge) and connects the domestic battery in parallel for charging and disconnects when the engine stops and the voltage falls.
The other solution as you have been told is to use diodes. These are as said like one way valves. You connect one from engine battery positive to domestic battery positive so current can only flow from engine battery (and charger) to domestic battery. However you will see that this will also allow the domestic lights etc to discharge the engine battery. So you need to fit 2 diodes. One from the alternator output to the engine battery and one from alternator to the domestic battery.
Now that can work. However as said the diodes drop a voltage about .7 volt. This drops regardless of current in a very non linear manner. (not like a light bulb) Unfortunately a drop of .7 volt in your regulated charging voltage will have a very dramatic effect on the charge of the batteries.
One solution is to use battery sense where the wire that takes a sample of the output voltage of the alternator is instead of normally sensing at the alternator output is taken to one of the batteries.
This will ensure that the voltage getting to the batteries will be 14v on charge. The voltage at the alternator output will have to rise to 14.7 volts to get the 14v at the battery because of the loss in the diodes. You can sense at either battery because they are both affected the same.
Unfortunately often the alternator has the regulator built into it and the sensing is done internally so it is difficult to dismantle the alternator find the sense wire and bring it out to a terminal. If you fit an after market smart regulator then these have a separate sensing wire so diodes are OK.
A final option is to use a special Schotky diode(s) which have a volt drop nearer .25 so cause less charging problems but still the charge will not be as much as it should be.
There are also I believe electronic devices that look like a diode with 0 volt drop which could also be used.
Or just use your switches wisely but i think VSR is best good luck olewill
I think as said a VSR is the best way to go
The concept is to provide a separated battery for domestic lights etc which is isolated from the battery used for engine starting. So that if you use lights too much you still have a battery to strt the engine to recharge batteries.
You can isolate the batteries and circuits very easily. The problem is that you must connect the batteries together to charge them when the engine is running (having only one alternator) and of course you must disconnect (isolate) the service battery when the engine is stopped. You can do this with switching (1,2,both switch) but you must remember to switch to both when engine is running then separate the batteries again when engine is stopped.
Now a VSR is a voltage sensitive relay which senses when the engine battery rises (which means it is on charge) and connects the domestic battery in parallel for charging and disconnects when the engine stops and the voltage falls.
The other solution as you have been told is to use diodes. These are as said like one way valves. You connect one from engine battery positive to domestic battery positive so current can only flow from engine battery (and charger) to domestic battery. However you will see that this will also allow the domestic lights etc to discharge the engine battery. So you need to fit 2 diodes. One from the alternator output to the engine battery and one from alternator to the domestic battery.
Now that can work. However as said the diodes drop a voltage about .7 volt. This drops regardless of current in a very non linear manner. (not like a light bulb) Unfortunately a drop of .7 volt in your regulated charging voltage will have a very dramatic effect on the charge of the batteries.
One solution is to use battery sense where the wire that takes a sample of the output voltage of the alternator is instead of normally sensing at the alternator output is taken to one of the batteries.
This will ensure that the voltage getting to the batteries will be 14v on charge. The voltage at the alternator output will have to rise to 14.7 volts to get the 14v at the battery because of the loss in the diodes. You can sense at either battery because they are both affected the same.
Unfortunately often the alternator has the regulator built into it and the sensing is done internally so it is difficult to dismantle the alternator find the sense wire and bring it out to a terminal. If you fit an after market smart regulator then these have a separate sensing wire so diodes are OK.
A final option is to use a special Schotky diode(s) which have a volt drop nearer .25 so cause less charging problems but still the charge will not be as much as it should be.
There are also I believe electronic devices that look like a diode with 0 volt drop which could also be used.
Or just use your switches wisely but i think VSR is best good luck olewill
I think as said a VSR is the best way to go
Jabs
Well-Known Member
I think you need to take some advice.
You may be talking about a split charging diode which is a PART of the circuit you may need.
It is fairly straightforward, but needs planning and careful execution.
Tony.
You may be talking about a split charging diode which is a PART of the circuit you may need.
It is fairly straightforward, but needs planning and careful execution.
Tony.
halcyon
Well-Known Member
The one problem with moving the sensing to the battery is, as said, the volt drop in a diode is non constant. A diode may drop 0.5 volt at low current, but at 70 amp it can be 1 volt plus, the 0.7 volt drop is only a nominal value.
This means that a service battery may be taking a high charge, droping 1 volt, the engine battery which is charged is only droping 0.5 volt. Now our alternator that regulates at say 14.4 volt, now has a 1 volt drop in it's sense lead, so increases output to 15.4 volt. The charged engine battery with it's 0.5 volt drop has 14.9 volt applied to it's terminals.
Connecting the sense to wire to the engine battery will stop this, but will give a lower voltage to service battery. That is, alternator is running at 14.4 volt + 0.5 volt = 14.9 volt. The service battery is on high amperage, so is droping 1 volt, so only gets 13.9 volt, not 14.4 volt. It gets more complicated, but is helped with Schottky diodes, but that's the basic, some firms list the problem as force charging.
Most Schottky high power diodes are around 0.5 volt drop on load, the voltage drop is lower on very low current though. If you find some down to 0.25 volt drop on full load, at a ecomical price, please PM me.
Brian
john_morris_uk
Well-Known Member
Brian, I run a split diode charging system with a Sterling 'smart' alternator controller sensing the domsetic battery bank and I used to worry about the implications of the theory that you have decribed so well above - but in practise there isn't an issue. This is because the internal resistance of the engine start battery rises and prevents it being over charged. I monitor the charge and voltages across both battery banks all the time and I have never had an issue.
The impression I get from all the replies to any question on spilt battery charging is that everyone who has a VSR says that they are the best, and anyone who has a split diode system with battery sensing says that theirs is OK as well!
My feeling is that split diode systems got a bad reputation from people who fitted them without battery sensing charging systems. The advatntages - as far as I can see, is that you eliminate the mechanical relay that is the VSR. Diodes mtbf (so long as they are not over run) is in the hundreds of thousands of hours - and I carry a spare split diode assembly on the boat at all times.
halcyon
Well-Known Member
It's that old story, or should it be a new thread, of how much do you want to spend, and how complicated you make the system.
Brian
ghostlymoron
Well-Known Member
I think the answer to this perenial chestnut is 'it depends where you're starting from' If you already have a diode setup with a smart regulator - that's fine and should give many years service.
However, if you are starting from scratch, the more modern VSR technology is cheaper and equally as good.
From the info below you can see that a diode setup will cost you £192 and a VSR setup £37 - £64 depending on the degree of complexity.
http://www.seamarknunn.com/acatalog/info_MTD160A2.html
Diode splitter £91
http://www.seamarknunn.com/acatalog/info_MTAR12VD.html
Alternator regulator £101
http://www.thetoolboxshop.com/0-727...nsitive-relay-for-charge-splitting-8096-p.asp
VSR £37
http://www.foxsonline.com/acatalog/BEP-710-VSR-125amp-Voltage-Sensitive-Relay-Dual-25276.html
BEP VSR £64 (dual sensing)
However, if you are starting from scratch, the more modern VSR technology is cheaper and equally as good.
From the info below you can see that a diode setup will cost you £192 and a VSR setup £37 - £64 depending on the degree of complexity.
http://www.seamarknunn.com/acatalog/info_MTD160A2.html
Diode splitter £91
http://www.seamarknunn.com/acatalog/info_MTAR12VD.html
Alternator regulator £101
http://www.thetoolboxshop.com/0-727...nsitive-relay-for-charge-splitting-8096-p.asp
VSR £37
http://www.foxsonline.com/acatalog/BEP-710-VSR-125amp-Voltage-Sensitive-Relay-Dual-25276.html
BEP VSR £64 (dual sensing)
PabloPicasso
Well-Known Member
Thanks to all the replies. It seems a vsr would be what I need.
DavidGrieves
Well-Known Member
Sorry didn't make my self too clear. Can't go open circuit due to human error. 1 2 both switch can get accidentally turned to the off position.