Blocking diode

[Slim]

Elsewhere on this forum it was suggested that the best way to reduce the 14.8v charge voltage from my Adverc controlled alternators (as recommended for my Lucas sealed AGM domestic batteries) down to about 14.2 for my sealed calcium engine start battery would be to use a blocking diode. I thought I had an old but unused 70a unit in the shed but I must have given it away.

 

Not wanting to spend £80+ on an encapsulated, screw to a bulkhead/connect wires unit (sorry, can't think of a better way to describe) I've been looking at stud diodes costing a couple of quid. However, what I don't know is how much heat they generate and what size heat sink would be required. Can anyone give me a steer either around the heat output or how to make up a unit. It would be mounted in the engine compartment (semi trad) so would need to be protected. I would also want it to be fairly neat and compact.

 

In fact I may want 2 but the second would only have to handle a notional 3a from the Mastervolt charger.

 

I do have a fall back option of replacing the start battery with a sealed AGM but would prefer not to go that way unless I have to.

 

My present set up is 2 x 70a A127 alternators controlled by Adverc Battery management system charging 5 x 120ah Lucas sealed AGMs (domestic) and 1 x 110ah sealed calcium (engine) split via 2 blocking diodes. (Adverc designed system). Presently the Adverc is set at around 14.4v but I intend to adjust it.

 

 

 

Frank

[ditchcrawler]

Nev on here had one for sale some time back that he took out of Percy when he changed some of the electrics. Or have a look here http://www.ebay.co.uk/itm/Land-Rover-Defender-90-110-Split-Charge-System-Diode-PRC4092-/131150133021?pt=UK_CarsParts_Vehicles_CarParts_SM&hash=item1e8928371d

[Slim]

Nev on here had one for sale some time back that he took out of Percy when he changed some of the electrics. Or have a look here http://www.ebay.co.uk/itm/Land-Rover-Defender-90-110-Split-Charge-System-Diode-PRC4092-/131150133021?pt=UK_CarsParts_Vehicles_CarParts_SM&hash=item1e8928371d

I did see that ad (or something similar) but wasn't too happy about the terminals. I'd be more comfortable with nuts and studs. Also, I'm sure I read somewhere, no idea where but in the last few days, someone commenting that they were prone to failing.

 

Anyway, thanks for looking

 

 

Frank

[Nickhlx]

Well, for dissipation, a silicon diode drops about 0.7 Volts, say 1 volt to err on the safe side, and if it is passing e.g. 3 amps, VI=W would mean that would be about 3 watts dissipation... Schottky diodes have a lower forward voltage drop so lower power dissipation at a given current passing, although that lower voltage drop may not be what you are wanting...

 

Nick

[rogeriko]

I would use at least 2 of these http://www.ebay.co.uk/itm/ZP-100A-Stud-Mounted-Rectifier-Diode-100A-1600V-for-Battery-Charge-/191274903332?pt=UK_BOI_Electrical_Components_Supplies_ET&hash=item2c88df6724 When the Chinese rate something at 100A they are actually good for about 30/40A. They will get extremely hot when passing the full 70 amps and must be bolted to a large heatsink at least 6 inches square with fins otherwise they will burn out. At high amps the voltage drop will be more than 1 volt but about .6v when the amps decrease as the battery charges.

Edited November 24, 2014 by rogeriko

[by'eck]

I would leave the charge voltage at 14.8 volts since the Adverc alt controller does not hold this voltage but cycles down to 14 volts. In fact the manual downloadable here suggests the voltage cycles between 14 and 14.4 volts, eventually settling at the lower voltage.

 

Even at 15 volts lead calcium batteries draw an incredibly low current when fully charged so there should be no gassing issues.

[keble]

Well, for dissipation, a silicon diode drops about 0.7 Volts, say 1 volt to err on the safe side, and if it is passing e.g. 3 amps, VI=W would mean that would be about 3 watts dissipation... Schottky diodes have a lower forward voltage drop so lower power dissipation at a given current passing, although that lower voltage drop may not be what you are wanting...

 

Nick

Just to add to this, having worked out the power being dissipated, the OP asked about the size of heat sink.

If you can get hold of the data sheet for the diode you should find advice on this.

Any electronic component will have a specified "thermal resistance". That means the temperature rise between the diode's working bits (junction) and the outside air per watt of power. The heatsink has to be capable of getting rid of enough heat to keep that internal temperature down to some reasonable level, say 100C.

If the temperature is too high the diode will fail " early".

Anyway, best thing is to get hold of the data sheet!

Edited November 24, 2014 by keble

[MoominPapa]

Elsewhere on this forum it was suggested that the best way to reduce the 14.8v charge voltage from my Adverc controlled alternators (as recommended for my Lucas sealed AGM domestic batteries) down to about 14.2 for my sealed calcium engine start battery would be to use a blocking diode. I thought I had an old but unused 70a unit in the shed but I must have given it away.

 

Not wanting to spend £80+ on an encapsulated, screw to a bulkhead/connect wires unit (sorry, can't think of a better way to describe) I've been looking at stud diodes costing a couple of quid. However, what I don't know is how much heat they generate and what size heat sink would be required. Can anyone give me a steer either around the heat output or how to make up a unit. It would be mounted in the engine compartment (semi trad) so would need to be protected. I would also want it to be fairly neat and compact.

 

In fact I may want 2 but the second would only have to handle a notional 3a from the Mastervolt charger.

 

I do have a fall back option of replacing the start battery with a sealed AGM but would prefer not to go that way unless I have to.

 

My present set up is 2 x 70a A127 alternators controlled by Adverc Battery management system charging 5 x 120ah Lucas sealed AGMs (domestic) and 1 x 110ah sealed calcium (engine) split via 2 blocking diodes. (Adverc designed system). Presently the Adverc is set at around 14.4v but I intend to adjust it.

 

 

 

Frank

ETA. Just read the whole post. Ignore my drivel.

 

MP.

Edited November 24, 2014 by MoominPapa