split diode charge unit

[wijnendael]

Hi all, I have a split diode unit fitted to the charging circuit on my boat that connects to two batteries from the alternator. I have measured the battery voltages when the engine is running and the main engine starting battery is showing just over 14 volt but the accessory battery is only showing 12.6 volt does this mean that the split diode is not operating correctly as this is around the same volts on the accessory battery as when the engine is not running.

 

[boatone]

Try measuring the outputs at the diode. Also, is there a long/inadequate cable run from the diode to the domestic? Check all connections are tight and clean.
Personally, I hate the things. They typically take about .7 of a volt for themselves.
You can measure the voltage drop by connecting the +ve of a DVM to the alternator output terminal and the -ve of the DVM to the output side of the splitter or to the battery +ve terminal.

 

[ms1]

I think it is well known that diode splitters loose about one volt between input and output. Good for sharing charge but not good at delivering full voltage required by the batteries for good charging.

 

[Nickcf]

[ QUOTE ]
...but not good at delivering full voltage required by the batteries for good charging.

[/ QUOTE ]

All depends on the design of the charger. If the battery voltage is sensed after the splitter ie at the batteries themselves, then there there isn''t a problem. If you just add a splitter to an existing charger then they won't charge fully.

 

[pheran]

The 'charger' in this case is the 12v engine-driven altenator. If that is putting out, say, 13.8v but the diode loss is 0.7-1.0v, the resultant 12.8v is insufficent to fully charge the batteries. I suffered this problem on a previous boat way, way back until I picked up a tip from a magazine (for radio hams!) and substituted a relay for the diode.

 

[muckypup]

I have the same problem. Loose about 1.0v with a split charge diode. It does while the engine running however produce enough charging capacity to power the plotter/radio/radar underway without it draining from the battery...

S.

 

[Nickcf]

[ QUOTE ]
The 'charger' in this case is the 12v engine-driven altenator. If that is putting out, say, 13.8v but the diode loss is 0.7-1.0v, the resultant 12.8v is insufficent to fully charge the batteries. I suffered this problem on a previous boat way, way back until I picked up a tip from a magazine (for radio hams!) and substituted a relay for the diode.

[/ QUOTE ]

Doesn't matter that it is an alternator charger. Alternator voltages are very high off load (which is why you must never isolate the batteries with the engine running as it will blow most 12v equpment) and the output is only regulated by the battery sense voltage and the control unit. If the sensing wire for the alternator (if it is accessible which it may not be!) is changed to the battery side of the splitter then full charging will occur. The only proviso is that the batteries are the same type and condition otherwise a faulty battery may drag down the voltage and hence cause overcharging. You don't need a relay if you can change the sense wire which is normally visible by the control unit as it is connected to the battery terminal.

nick

 

[wijnendael]

Thanks for all the replies, I will check the voltage drops to the batteries to try and determine if this is the problem. I am not sure without checking where the sense wire is connected until i look. Lastly one factor may be after reading the last post, the batteries installed are not the same size (physical) the battery for starting the engine is a normal size battery same as in a car. The accessory battery is twice the physical size of the engine battery does this make a difference ?

 

[Nickcf]

[ QUOTE ]
... Lastly one factor may be after reading the last post, the batteries installed are not the same size (physical) the battery for starting the engine is a normal size battery same as in a car. The accessory battery is twice the physical size of the engine battery does this make a difference ?

[/ QUOTE ]

Have a look at http://www.smartgauge.co.uk/diodes2.html

Check what the battery capacities are (ie the ampere hours etc). If they are very different I wouldn't recommend using a splitter. If the ampere hours are similar then connecting the sense wire to the battery with the most load will ensure that the alternator is supplying the current rather than the battery. However if one battery is nearly flat I wouldn't attempt to charge using a diode splitter as there is a chance you will over charge the other (good) battery.
Bottom line is diode splitters have their uses but are not the best solution for different batteries. Using a smart charge controller would be the best solution (no connection with the web site by the way, just thought their write up was useful)

 

[boatone]

Ahh well, if the batteries are different sizes that alters things! The domestic battery is probably a 110AH or thereabouts whilst the engine starter battery may only be around 60Ah.

My simple solution to this would be to chuck out the diode altogether and connect the output from the alternator to a 1-Both-2 switch with the engine battery connected to the 1 terminal and the domestic battery connected to the 2 terminal. You can then decide which battery you want to chage and for how long. In practice I would charge the engine battery for a while to restore starting drain but keep the domestic on charge for the bulk of the time. That way there is no voltage lost to the diode splitter at all and you can prioritise the charging regime depending on circumstance/need. However you nust remember not to start the engine (or have it running) without the switch being ON in one position or another.

Also not a bad idea to wire in a permanent 'jump lead' between the two battery positive terminals with an isolation switch in the middle. That way its easy to temporarily connect to the domestic battery if you need emergency starting power

 

[Observer]

Serious question. I'm not an expert but I keep reading this stuff about split charge diodes being 'no good' because of voltage drop etc.

If that is true, what is wrong with the contents of this guide which, as far as I understand it, is saying that a split charge isolator (which is based on diodes) is the best way of charging multiple battery installations? (Obviously they have a vested interest in the products they manufacture but surely the technical arguments/justification are either valid or not?)

The set-up on Fair Prospect uses two of these Surepower isolators to charge four separate battery banks (engine, domestic, bow thruster, stern thruster), all of which are charged from either or both engine alternators and the Mastervolt battery charger.

I think the isolators had the secondary valuable function of preventing damage to the charger and the engine alternators recently when an electrician, who had changed one of the thruster batteries, connected the diode isolator output lead to the negative of the new battery instead of the positive.

I'm seriously interested to understand more about this.

 

[Nickcf]

[ QUOTE ]
.....I'm seriously interested to understand more about this.

[/ QUOTE ]

have a look at http://www.smartgauge.co.uk/diodes2.html which may help.

 

[pheran]

Like you, I'm no expert but I don't think anyone is saying there is anything wrong with the splitting diode solution. Its a very straight-forward, simple way of doing the job thats been around a very long time (NB the copyright on that document dates from 1972 and there have been no updates for at least 13 years)
But the fact is a diode, measured on the bench, will drop the voltage by 0.7v. So in practice, the loss is likely to be nearer 1.0V. If you have an application that can afford this loss, all well and good but most automotive/boat charging systems can't especially when added to the other incremental losses that exist in the system. The result is undercharging and batteries that are never up to full capacity with obvious immediate results and reduced battery life in the longer term. These days there are no doubt smart charging systems that can do something to allieviate this especially when combined with high output altenators. All at a cost of course but the spitting diode does at least have the advantage of being cheap.

 

[Observer]

[ QUOTE ]
[ QUOTE ]
.....I'm seriously interested to understand more about this.

[/ QUOTE ]

have a look at http://www.smartgauge.co.uk/diodes2.html which may help.

[/ QUOTE ]

To be honest, there's a lot of waffle on that site (mostly knocking split charge diodes and mostly because of the voltage drop) which may or may not be valid (I don't know, I'm not an expert). There's not that much which explains why a relay system is inherently better. Particularly the issue (as I understand it) that the relay is binary - different battery banks are either electrically connected or not. And once they are, a heavily discharged battery can drag down (even damage) a better one.

I may have the wrong end of the stick - as I said, I'm not an expert. However, posting a link to something I've already read, that doesn't address the question I asked - "What is wrong/invalid about the content of the Surepower guide", doesn't improve my understanding, I'm afraid.

Take this comment from the Smartgauge site:

[ QUOTE ]
A split charge diode only splits the charge from the alternator. If a wind turbine, or solar panel, or single output charger is added, yet another split charge system has to be added.

[/ QUOTE ]

So how does my installation manage to split the charge from two alternators plus the battery charge to four separate battery banks?

 

[halcyon]

How does it work ?

Firstly the engine is always connected directly to it's battery, thus if the split charge system fails you keep charge to engine battery. Thus there is your charge to two banks.

Now talking slmplisticly, a battery has three voltage ranges, off charge / open circuit, on load, and on charge. A flat battery on charge is approx 13.0 volt, a fully charged battery is over 15 volt, on discharge, a full battery is around 12.5 down to 10.8 volt. The system now monitors engine battery voltage for a set point, we use 13.8 volt now, at this point the relay is engaged. The engine and service battery are now connected in parallel. This results in a volt drop, as charge takes the least line of resistance, and charge voltage is set by this circuit. Normally the service battery is above 50% charge, and this drop is somewher above 13.0 volt. This voltage is below the current engine charge voltage, but above the discharge voltage. So all alternator output flows to the service battery, as it's to low to charge the engine battery, and to high to discharge the engine battery. Once the charge voltage catches up with charge voltage of the engine battery, they both start taking a charge current relative to there state and capacity.
If voltage falls due to no / low charge of the alternator, voltage falls to the no charge level , 13.0 volt, and the relay will drop out.
By using differant voltage trip levels you can cascade the charge load on the alternator, i.e. charge service battery, then bow battery and so on, rather than hitting it with all loads in one hit.
By using a bi-directional charge system, that is monitoring battery voltage on both sides of the relay, you can provide alternator redundancy for twin engines, and allow a second charge source on the service battery to charge the engine.

Life gets more complex if you go on, as you can include, link starting for flat engine battery, isolation of a flat engine battery during link start, battery temp monitoring and cut out, over current on charge circuit drop out, i.e. winch bow thruster load.

Does that help. it's easier to explain verbally, than in writing.

Brian

 

[Observer]

Thanks for that. I think I follow but it does seem quite complicated compared to the diode/isolator solution.

Do you have any idea of the comparative cost of the relay solution vs diode/isolator for an installation with four battery banks and three charging sources (two alternators and battery charger)?

 

[boatone]

The originator of this thread has a much simpler sytem than yours with only two batteries, one for starter and one for domestics, and a single alternator.

Your setup is much more complex and I would expect it to have some form of battery management system such as those from Stirling or Adverc which monitor the state of charge and charging voltages.
These systems also sense the voltage reaching the battery terminals and adjust alternator output accordingly, so any losses due to diodes in circuit would usually be compensated for to ensure the optimum charging voltage is maintained.

My earlier advice re a 1-both-2 switch was also predicated for the original posters needs. I would not presume to advise on your setup !

Incidentally, if you have a battery management or related problem I have found the guys at Adverc really helpfull, even if you don't actually buy anything.

 

[Observer]

Surepower charging systems guide said:
[ QUOTE ]
if two or more batteries are to be connected and charged in parallel, they must be the same size and age. If not, one battery or the other may be damaged.

[/ QUOTE ]

Smartgauge technical description said:
[ QUOTE ]
SmartBank essentially monitors two battery banks and if either bank is being charged it parallels both battery banks.

[/ QUOTE ]

Surepower charging systems guide said:
[ QUOTE ]
There are many switches, relays, solenoids, and other variations of switches on the market that claim to be battery isolation devices. This CANNOT be! IT’S THE LAW! These cannot isolate; they simply disconnect one battery from the other when "OFF," but reconnect the batteries when turned "ON". In the "ON" position the full battery can discharge at an uncontrolled rate into the low battery. If the low battery is substantially lower than the other battery, this current can be so high that the wiring, plugs, and the switch contacts can overheat, creating a possible fire danger

[/ QUOTE ]

Putting aside the flowery language in the Surepower guide, the underlying statements seem to be:

- if battery banks are connected in parallel (e.g. by a relay based split charge system), the batteries should be the same capacity and age

- if there is a substantial difference in state of charge between two batteries connected in parallel (e.g. by a relay based split charge system), the charged battery will discharge to the discharged one in an uncontrolled way

If the Surepower statements are correct, the relay system (despite the advantage of no voltage drop) has a significant drawback. I understand that a relay system can incorporate a low volt drop out circuit that disconnect the batteries if one is substantially discharged and the other has a high charge. That eliminates the risk of "charge feedback" (is that the correct term?) but surely leaves the discharged battery without means of being charged.

And the relay system still doesn't address the problem (if it is a problem) of different battery capacities (either nominal or because of age).

 

[halcyon]

What are your actual battery banks used for ?

Is the battery charger a single output?

A relay solution envolves less work in installation and mods to the system than diodes, and gives you full alternator voltage, without having to fit smart regulators as well.

Brian

 

[Observer]

[ QUOTE ]
What are your actual battery banks used for ?

[/ QUOTE ] engine battery, domestic/service battery, bow thruster, stern thruster

[ QUOTE ]
Is the battery charger a single output?

[/ QUOTE ] yes


I'm not sure I have a smart regulator fitted. It's nowhere I've seen it and not shown on the wiring diagram.