Split charge diodes versus split charge relays

[catmandoo]

While trawling the site trying to find a replacement Compensating diode for the alternators feeding my Sure Power battery isolators I stumbled onto an apparent controversy between the various factions promoting what appears to be one group of equipment manufacturers products versus another eg Sterling versus Smartguage as one example

While no doubt each group has a case to defend it strikes me that the dangers they warn about o only occur in certain circumstances which most users might never see due to their modes of operation and also because of the inbuilt inefficiencies of their systems to fully charge all batteries

Cases against the diode installation include the tendency to overcharge the starter battery and loss of energy as heat across the diode banks
Cases against the relay installation are that it is dated and dangerous especially if an inverter is used and more so as a relay is prone to vibration faults and overloading .
A relay is easy to fit however and can be used to charge a bow thruster battery .

Looking back at history we had an alternator feeding a starter battery inherited from the car industry , then we added more batteries when we switched from gas and kerosene to power fridges and lamps . Because we now had more batteries and wanted to distribute charge proportionally and not allow one battery to drain another we added relays and /or diodes . After that its Inverters , solar panels , wind turbines etc. It therefore seems to me that the current set up is dictated by history and that if users, builders and equipment manufacturers were to start with a clean sheet of paper looking at the systems as a whole and working together we might end up with a simpler integrated system rather that just a collection of addons , .


Any ideas ?

 

[Stu Jackson]

The relays and diodes only came about because people didn't understand how to use a simple switch!!!

 

[ianj99]

While trawling the site trying to find a replacement Compensating diode for the alternators feeding my Sure Power battery isolators I stumbled onto an apparent controversy between the various factions promoting what appears to be one group of equipment manufacturers products versus another eg Sterling versus Smartguage as one example

While no doubt each group has a case to defend it strikes me that the dangers they warn about o only occur in certain circumstances which most users might never see due to their modes of operation and also because of the inbuilt inefficiencies of their systems to fully charge all batteries

Cases against the diode installation include the tendency to overcharge the starter battery and loss of energy as heat across the diode banks
Cases against the relay installation are that it is dated and dangerous especially if an inverter is used and more so as a relay is prone to vibration faults and overloading .
A relay is easy to fit however and can be used to charge a bow thruster battery .

Looking back at history we had an alternator feeding a starter battery inherited from the car industry , then we added more batteries when we switched from gas and kerosene to power fridges and lamps . Because we now had more batteries and wanted to distribute charge proportionally and not allow one battery to drain another we added relays and /or diodes . After that its Inverters , solar panels , wind turbines etc. It therefore seems to me that the current set up is dictated by history and that if users, builders and equipment manufacturers were to start with a clean sheet of paper looking at the systems as a whole and working together we might end up with a simpler integrated system rather that just a collection of addons , .


Any ideas ?

The ideal solution would be a microprocessor based controller with:-

1) inputs for 1 or 2 alternators, solar panel, wind geny and mains charger.
2) It would have 2 or more outputs for starter and house batteries.
3) It would be able to monitor the state of each battery and decide which receives priority.
4) It would alert you to a fault, and warn of low battery voltage.
5) It would use contactors rather than relays, diodes or mosfet switching to eliminate voltage drop losses.
6) It automatically prioritises the inputs in order of current supplying ability and current demand. (eg switches to solar or wind and switches off the mains charger or alternator when demand is low enough)
7) It would include digital charge regulators for all the input devices - directly controlling the alternators for example.

Now for the cons:
8) It would need to come with 4years interest free credit for you to be able to afford it!! Which is why no boat builder would be interested.
9) If it went on the blink - no charging at all.

So having separate non integrated systems can be the best and most cost effective after all..!

Ian

 

[catmandoo]

The relays and diodes only came about because people didn't understand how to use a simple switch!!!

Well its all clear if you use the water analogy . Switch = tap , battery = pail .

However if you are using switches when you have 4 batteries you must have 4 switches i.e. four taps but with one source the alternator . Theoretically then when each bucket is full you turn off it's tap . But with some half full , half empty or full at any one time you have to be selective . Taps ( switches ) are just on off devices so you control the flow in short bursts assuming you must have a reliable means of working out the level of water in each bucket( voltage ?) at any one time and allowing for foaming at the top by increasing the frequency of operation when the bucket is almost full

On problem mentioned before is that the out let of each pail is connected to a common distribution header so if one is fuller than another in some circumstances it flows into the lower one by the back door so to speak

You measure the fullness of each bucket by its water level (head ) = voltage but unfortunately control the input not at the taps but at the water source ( the alternator) measuring the fullness only at the bucket with the biggest use again by an on off control loop so when that battery gets full first the others suffer

All this takes time and when you start to sail the engine goes off and the power source stops regardless of whether the batteries are full or not .

Despite today's technological expertise in other areas we use a control system that was developed in the dark ages and to be silly equivalent to a trained monkey turning a tap on and off when the bucket got full with a banana for the motivation control


It seems to me that neither the switch or the diode system is ideal and we are just content to muddle along with expensive add ons to suit various manufacturers instead of devising an efficient , simple fool proof system from scratch using a bit of synergy from other areas of system control to come up with a good all round design with minimum wires and black boxes KISS

its probably out there and working and just that the rest of us don't know about it

 

[jimbaerselman]

You were looking for a battery to battery charger

 

[GeorgeTina]

I have used a mosfet splitter for the last seven years with no problems. The volt drop at 50amps charge is .05 volts so no need to compensate for volt drop. Both batteries are charged independently. Very simple solution.

 

[VicS]

I have used a mosfet splitter for the last seven years with no problems. The volt drop at 50amps charge is .05 volts so no need to compensate for volt drop. Both batteries are charged independently. Very simple solution.

"Very simple solution" except that if you go to your favourite chandlery catalogue or website you are unlikely to find anything called "Mosfet splitter"

An electronics catalogue will find you little electronic components called "mosfets" but that is not a lot of help!

 

[rogerthebodger]

I think the Vetus an the X-split by Driftgate are both mosfet as both have a negative input which normal diodes do not need but mosfet's do

 

[catmandoo]

You were looking for a battery to battery charger

yes frustrating isn't it . Makes all that other gear we bought redundant !?

 

[catmandoo]

I have used a mosfet splitter for the last seven years with no problems. The volt drop at 50amps charge is .05 volts so no need to compensate for volt drop. Both batteries are charged independently. Very simple solution.

so where did you get it as a unit? PLEASE

 

[GeorgeTina]

Try http://www.barden-uk.com/battery-splitters.html
for example. I can't remember where I got mine - sorry.
I am sure you can see more if you google search.

 

[catmandoo]

Try http://www.barden-uk.com/battery-splitters.html
for example. I can't remember where I got mine - sorry.
I am sure you can see more if you google search.

Could not get into Barden site but found another at Advanced yacht Systems . Interesting . Thought Sterling did a similar unit but they just use normal diodes claiming that theirs are better that others .

Interested in reliability though . Anybody else with Mosfet experience ?

 

[halcyon]

The ideal solution would be a microprocessor based controller with:-

1) inputs for 1 or 2 alternators, solar panel, wind geny and mains charger.
2) It would have 2 or more outputs for starter and house batteries.
3) It would be able to monitor the state of each battery and decide which receives priority.
4) It would alert you to a fault, and warn of low battery voltage.
5) It would use contactors rather than relays, diodes or mosfet switching to eliminate voltage drop losses.
6) It automatically prioritises the inputs in order of current supplying ability and current demand. (eg switches to solar or wind and switches off the mains charger or alternator when demand is low enough)
7) It would include digital charge regulators for all the input devices - directly controlling the alternators for example.

Now for the cons:
8) It would need to come with 4years interest free credit for you to be able to afford it!! Which is why no boat builder would be interested.
9) If it went on the blink - no charging at all.

So having separate non integrated systems can be the best and most cost effective after all..!

Ian

Yes we can offer most of your list now

The diode / relay problem is due to most people not understanding how or why the VSR system was developed 32 years ago.

A bit of pre-Sterling/Smartgauge history.

Diodes go back to 1960's so have been with us many years, blocking diodes back then had a number of problems,

1) The alternator output had to go through the diodes, thus a diode fault could isolate the alternator from the engine battery, resulting in no charge, open circuit alternator, and a flat starter battery.
2) The diode volt drop caused severe reduction in charge levels.

The VSR system was developed to avoid the diode faults,

1) Alternator always remained connected directly to the engine battery, even if there was total VSR failure.
2) Zero volt drop, well 0.04 volt @ 100 amp.
3) Bi-directional battery charge, engine to service, or service to engine, this allowed a mains battery charger on the service battery to also charge the engine battery.
4) Low battery, a VSR can also monitor low voltage, so power can be cut off at low voltage, isolating non-critical equipment to reduce drain till recharge, thus extending battery life.

5) more up to-date, the relay can be manually controlled, so requires no electronics, or even electric supply.


MOSFET's have the same problem as diodes, the alternator output must go through the split charge unit. So a unit failure means no charge to engine battery.
The second reason I have not use MOSFET's is they have a internal flywheel diode. This means you can switch current between A & B, and stop current flowing from A to B, but the diode allows current to flow from B to A.
Plus you cannot use bi-directional charge, or emergency link start, and the volt drop is no better than a relay.

Most other points you read in recent years are the result of marketing.


Brian

 

[William_H]

Battery charging

The obvious simple system is where you have a diesel engine that can be hand crank started. So just one battery for everything. If you do flatten it hopefully there will be enough charge left to excite the alternator once the engine has started.
If you can't hand start then you will want a quarantined engine battery to start the engine which can not be discharged by careless use of domestic power by having a separate domestic battery. This has the other advantage that when and if the engine battery fails you have a second chance in using the hopefully not discharged domestic battery.
Having the engine battery separate and isolated means that for charging the domestic battery off the engine you must connect it to the alternator which is in effect connecting it in parallel to the engine battery.
You can do this with switches. You just have to remember to connect the domestic battery on engine start and disconnect on engine stop.
The 1,2,both switch is an alternative system where the entire system (engine start and domestic) is connected together as one but an alternative battery is available. You can charge both on "both" or charge one at a time. The plan is that the 2 different batteries be used at different times so charging them both and confirming both are good. (for engine start) Unfortunately the lazy find it easiest to start and run on both. This gives you double the battery capacity but still only one "battery" in that they can both be inadvertently discharged. Or worse one can die and the other one masks the effect.
The diode system seems ideal. The alternator output is fed to the anodes of 2 diodes the cathodes of which go to each battery positive. The diodes provide total isolation except that current can flow from the alternator to the batteries. Diodes however inherently drop a voltage. For silicon diodes this is .7 volt at low current increasing at higher currents. This volt drop means that an alternator whose output voltage is sensed and so regulated at the alternator output will never be able to fully charge the batteries.
If the sensing wire of the regulator can be removed from the output of the alternator and moved to one of the batteries the end result voltage is sensed so output of the alternator is increased to allow for the diode drop. Another disadvantage is that at 10A charge the diode pair will dissipate 7 watts which will make it hot and waste power. An alternative would be to increase the regulator voltage and or use Schotky diodes which drop something like .25 volt at low current.
A far simpler system is to use a Voltage Sensing Relay. This is just an ordinary relay which connects the domestic battery to the engine battery.It is however powered by a clever circuit which senses when the engine battery voltage has risen to typically about 13 volts. This means the engine battery has received some charge first before the domestic battery. This same circuit disconnects the relay when the engine (and domestic battery combined) voltage falls below 13volts. Which is what happens when the engine is stopped.
Many say the relay is a mechanical switch which is unreliable compared to semiconductor diodes. Relays can however be very reliable when sealed in nitrogen.

As said the relay contacts can be emulated by using MOSFET transistors. These have a very low on resistance and no inherent volt drop. They do unfortunately need a higher voltage +ve than the 12v line they are in if it is a usual MOSFET type. So in addition to the voltage sensing circuit you need a switch mode voltage multiplier to turn them on. Also as said most MOSFETS have a built in diode which will bypass reverse current which is exactly what you don't want. However these problems have been overcome in commercial design.

So I disagree in that the modern dual battery system can be pretty good and simple. It can be enhanced by programmed charging regulators to give more charge from a shorter engine run. (smart chargers)
Using a smart charger regulatror does mean the sensing voltage wire can be taken to one battery so making diodes a practical solution leaving only the power wastage as the small disadvantage.
good luck olewill

 

[ghostlymoron]

The relays and diodes only came about because people didn't understand how to use a simple switch!!!

Whilst most people can understand how to use a simple switch, many will sooner or later forget and, it's in that situation that a volt sensitive relay comes into it's own in that it prevents you having 2 flat battery banks. In my opinion it's a no-brainer - fit one! This whole argument has been rehearsed on this forum and elsewhere and the conclusion is always the same but still you will get the 'flat earthers' who insist that a 1-2-B-O switch is all you need.

 

[sailinglegend420]

You were looking for a battery to battery charger

I'm surprised you off all people Jim have pointed readers to this Sterling product!

This would appear to be the ideal solution but even Mr Sterling warns you of its dangers on page one - and I quote - "For best effect use open lead acid batteries, avoid gel, sealed and AGM batteries."

You have been warned - again!

Split diodes have one other problem not yet mentioned. By having regulators that sense the service battery voltage and boosting the charger output you can produce the right voltage at the service battery but this can produce too high a voltage on the starter battery. Consider a heavily a depleted large service bank - and an almost full smaller starter battery. The voltage drop across the diode feeding the service bank increases with the current so may well reach 1.4 volts or more. The alternator senses the voltage at the bank and ups the output by 1.4 volts to say 15.8 volts. The voltage drop across the diode feeding the starter bank is only 0.7 volts because the smaller service bank is taking a much smaller current. So the starter bank is sitting at 15.1 volts for maybe a few hours! This is almost an equalizing charge - so not very good for Gel, sealed and AGM batteries.

 

[catmandoo]

split charge diodes

I'm surprised you off all people Jim have pointed readers to this Sterling product!

This would appear to be the ideal solution but even Mr Sterling warns you of its dangers on page one - and I quote - "For best effect use open lead acid batteries, avoid gel, sealed and AGM batteries."

You have been warned - again!

Split diodes have one other problem not yet mentioned. By having regulators that sense the service battery voltage and boosting the charger output you can produce the right voltage at the service battery but this can produce too high a voltage on the starter battery. Consider a heavily a depleted large service bank - and an almost full smaller starter battery. The voltage drop across the diode feeding the service bank increases with the current so may well reach 1.4 volts or more. The alternator senses the voltage at the bank and ups the output by 1.4 volts to say 15.8 volts. The voltage drop across the diode feeding the starter bank is only 0.7 volts because the smaller service bank is taking a much smaller current. So the starter bank is sitting at 15.1 volts for maybe a few hours! This is almost an equalizing charge - so not very good for Gel, sealed and AGM batteries.

Well I have read all the comments and while interested in the Mosfet splitters I think if I can get a replacement compensating diode then I will stick with my original system which has been running without a fault for the last 27 years

Those advocating relays seem to be involved in their installation so i suspect they are electrically biased

As for the starter battery being overcharged i can't understand that logic since if it is already at a higher voltage that the others then it all depends on the relative total voltage drop through each of 3 circuits which in my case serve 4 by 100 amp batteries Even with 50 amps maximum belting through the system that reduces to a max of 25 amps at each of my 1 domestic and 2 power batteries in parallel and I suspect there is Delta V between the alternator and the service batteries to keep the flow in that direction and leave the diode serving the starter in shut mode as nothing untoward has happened to date . If I have the time I will do several calculations assuming the worst cases to see what happens . In the end as always its the sums that speak the loudest in any argument backed up by what happens in practice . Also with temperature sensing feeding the smart chargers from the batteries the risk is even less

Also even if my starter battery was knackered I could still use my no 2 battery for starting as the battery selector switch was engineered to be down stream of the diode units

 

[VicS]

I think if I can get a replacement compensating diode

What is the compensating diode and what does it do?
What is special about it?

 

[jimbaerselman]

This would appear to be the ideal solution but even Mr Sterling warns you of its dangers on page one - and I quote - "For best effect use open lead acid batteries, avoid gel, sealed and AGM batteries."

You have been warned - again!

Well, I'd recommend using only open lead acid batteries for cost effectiveness, particularly if you're spending lots of time in foreign places. But that's another discussion.

The battery to battery charger is only a sophisticated relay. One which charges the engine battery first. When that's charged, and goes to float (say 13.6) sophistication cuts in. The charger then takes current from the engine battery to charge domestics, but amplifies the voltage as needed. Your alternator then has to raise current output to keep the starter battery at float.

The combination fits KISS principles nicely. And, yes, you'd need carry some de-ionised water.

 

[catmandoo]

What is the compensating diode and what does it do?
What is special about it?

machine sensed alternators only measure voltage at alternator out put so with split charge diodes fitted between the alternator and the battery you have to fool the alternator in thinking that it is reading battery voltage by fitting a diode between the auxiliary terminal and the regulator to compensate for the voltage drop across the split charge diodes . This has been the state of technology since at least 1990 when mine were fitted by a Swiss Electrical Engineer when he owned the boat and they have not failed since. However with the advent of the smart charger there is an opportunity to try to get more into the batteries

With smart chargers I understand you don't need the local regulators and machine sensing as the smart units measure battery voltage directly. its only if the Smart charger fails for any reason that I want to have the back up

The draw back with the diode system seems to be that there is a significant heat loss and in these days of high fuel costs etc its possibly worth trying to do away with the loss by fitting a Mossfet unit which has low Delta V which I am currrently considering substituting for the Sure Power unit/s

As my boat is in Turkey with high ambient temperatures I am also keen to reduce any local hot spots