Split charge relay gets quite warm: 3 questions

[RedGus]

Sorry to labour the point, but battery charging doesn't really work like that. Once the engine has started, the relay connects the two batteries together, so they are acting as a paralleled bank. For example, if you have a 60AH starter battery and a 60AH leisure, once the relay energises you effectively have a 120AH bank of batteries. The way that lead acid batteries under charge work is that they are self-limiting for charge rate. So, if you have two in parallel, and one is moderately discharged (usually the starter) and the other heavily discharged, you will have a situation where the heavily discharged battery will be taking almost all of the available charge. This means that you may well find a situation where the charge required by the battery is everything that the alternator can supply. The only reason to have a fuse in the circuit is to protect the wiring or the components from being damaged by a current that is too high. If your cable between the two batteries is correctly sized, it really doesn't make sense to use a 70A relay, then fuse the cable at 30A when there is a potential charge current of 55-65A. After all, isn't the main aim to charge the batteries with all that the alternator has to offer, as quickly as possible?

I'll get me coat.

[kevtherev]

Not really the alt output is 65 amp poss 10 to run engine 55 left split between 2 batts one just started the engine the other run for van leisure chances are it will never blow as both req a good charge.

As said.. charging unequally discharged batteries is not like that... the deep cycled batt will haul away as much current as it can

[California Dreamin]

Yes, my point exactly, it should be connected to a relay that works the same as a split charge, NOT to the split charge relay.

Agreed.... and that is why several manufacturers (Lucas/Durite) sell a split charge relay that has a extra fridge terminal output (live only with the engine running) or I suppose you could spur the same trigger wire used on the split charge relay upto a separate fridge relay.

http://www.autoelectricsupplies.co.uk/product/824" onclick="window.open(this.href);return false;

Martin

[kevtherev]

or I suppose you could spur the same trigger wire used on the split charge relay upto a separate fridge relay.

which is how mine is rigged by autosleeper, except the trigger is able to be switched off

[California Dreamin]

Sorry to labour the point, but battery charging doesn't really work like that. Once the engine has started, the relay connects the two batteries together, so they are acting as a paralleled bank. For example, if you have a 60AH starter battery and a 60AH leisure, once the relay energises you effectively have a 120AH bank of batteries. The way that lead acid batteries under charge work is that they are self-limiting for charge rate. So, if you have two in parallel, and one is moderately discharged (usually the starter) and the other heavily discharged, you will have a situation where the heavily discharged battery will be taking almost all of the available charge. This means that you may well find a situation where the charge required by the battery is everything that the alternator can supply. The only reason to have a fuse in the circuit is to protect the wiring or the components from being damaged by a current that is too high. If your cable between the two batteries is correctly sized, it really doesn't make sense to use a 70A relay, then fuse the cable at 30A when there is a potential charge current of 55-65A. After all, isn't the main aim to charge the batteries with all that the alternator has to offer, as quickly as possible?

I'll get me coat.

This has been explained a few times but continues to be miss-understood....RedGus is half correct....whilst two batteries are connected together (as in a split charge circuit) the alternator does see them as one and yes, the leisure battery which is generally more discharged will take the majority of the current. However, this is where the confusion lies. The alternator's regulator is not smart enough to know that there are two batteries, it senses PD (potential difference) between the regulators output of approximately 14 volts and the 'OVERALL' battery voltage (now this is important). It effectively ADDS the values of the two batteries together...for example: 11.9 volts leisure battery (40% charged) and 12.7 volts (starter 100% charged)...So the regulator sees the medium point of around 12.3 volts (approx. 70% charged) and sets about replacing that deficit. As the sensed voltage increases the PD (potential difference) decreases which in practice means alternators initially 'BULK' charges (charges with a high current) but reduces the output to almost a trickle in the last 25% of the batteries charge cycle.
The Leisure battery just never gets fully charged.
If you rely on a traditional split charge system as the SOLE means of charging a leisure battery you can effectively 'right off' 20 - 30% of your leisure batteries capacity because of this phenomenon. However, the easiest way around this is to fit a mains leisure charger and use the van on hookup on a regular basis.



The alternators 'Charge Curve' isn't stepped but infinitely diminishing....it will charge to 50% very quickly (Bulk), perhaps just 10 - 15 minutes but then take an hour (as the output decreases) to take this to 75% and then several hours to reach 100% . Then consider the fact that it see's ONE big battery (when two are connected together) the PD is an average of the two and so the second often larger leisure rarely achieves it's full capacity but rather ends up around the 70 - 80% mark.


Martin

[ghost123uk]

And lets not forget, if the leisure battery is heavily depleted, as soon as the split charge relay is activated, the starter battery (which is not depleted despite a 2 second bust on the starter motor) supplies current to the low voltage of the leisure battery (regardless of what the alternator is doing) this current can be very high (ok, only for a short period, but it still happens).

~~~~~~~~~~~~~~~~~

I think there are some "crossed lines" above re fridge supply and 5 pin split charge relays Just in case it needs confirming = if your fridge is connected to a 5 pin split charge relay,(correctly), it will only come on when the engine is running and it is being fed from both batteries and the alternator (as under those conditions those parts are all connected in parallel).

"crossed lines" = almost a pun

[RedGus]

At the risk of turning this into a rather more complex thread than the OP perhaps intended with the "warm relay" question, there are a few things to add to the post from California Dreaming above. The problem with charging leisure batteries from a vehicle alternator really stems from the fact that the original alternator is designed simply to replace the small amount of charge used to start the engine, drawn from a battery at a high state of charge. Leisure batteries get much more highly depleted in use, so require a different charging regime, as CD says. Where I beg to differ slightly from what you say, CD, is that the voltages you referenced are for a battery which is off-charge and under no load. In practice, as soon as the engine starts, the voltage from the alternator will immediately rise above 13V, even for a heavily depleted battery. Remember, each battery will take what current it needs, until it is "full", and it is the battery that effectively controls this taper-off of charge rate, not the alternator. All the regulator does on the alternator is to ensure that the output voltage does not rise above a preset value (usually between 13.8 and 14.4V). Until the battery (or batteries, with a leisure battery in the equation) terminal voltage has reached the regulator voltage, the alternator should be charging at a high current (the "Bulk Phase" referred to by CD). After this preset voltage has been reached, the current starts to taper off - the "Absorption Phase", if you're interested.

The best way to ensure that batteries get the best charge they can, on the vehicle, and a very simple fix, is to change the alternator regulator for one with a higher output voltage. Many of our original alternators will still have their 13.8V regs. Simply changing this for a 14.4V one will make a HUGE difference to how well your leisure battery charges. I have a fair bit of experience with boat charging systems, where there is usually a very large domestic battery bank, and the need to recharge as efficiently as possible. Much has been written about this subject by people who are real experts, and the consensus is that the best thing you can do to ensure the fastest charge and the best chance of getting the state-of-charge closer to 100%, is to raise the charging voltage to just below the gassing point of the batteries. This will be 14.4 - 14.6V for the majority of lead acid batteries. The majority of boaters have no access to mains hook-up to externally charge their batteries, but manage a very efficient charge regime by using the alternator mods I've just described, combined with some solar charging for the final part of the Absorption Phase, which can often take a LONG TIME to complete, at much lower current.

One final thing to watch: try never to let your leisure battery to get below 50% state-of-charge (or about 12.2V under no load). Repeatedly pulling batteries down to a very low state-of-charge, where the lights go dim, etc, is a sure-fire way to destroy expensive batteries well before their time.

[kevtherev]

Thank you redgus interesting reading.
I have been banging on about traction batteries for a while now
I'd be interested if you have any opinions about them.

[California Dreamin]

At the risk of turning this into a rather more complex thread than the OP perhaps intended with the "warm relay" question, there are a few things to add to the post from California Dreaming above. The problem with charging leisure batteries from a vehicle alternator really stems from the fact that 1) the original alternator is designed simply to replace the small amount of charge used to start the engine, drawn from a battery at a high state of charge. Leisure batteries get much more highly depleted in use, so require a different charging regime, as CD says. Where I beg to differ slightly from what you say, CD, is that the voltages you referenced are for a battery which is off-charge and under no load. In practice, as soon as the engine starts, the voltage from the alternator will immediately rise above 13V, even for a heavily depleted battery. Remember, each battery will take what current it needs, until it is "full", and 2) it is the battery that effectively controls this taper-off of charge rate, not the alternator. All the regulator does on the alternator is to ensure that the output voltage does not rise above a preset value (usually between 13.8 and 14.4V). 3) Until the battery (or batteries, with a leisure battery in the equation) terminal voltage has reached the regulator voltage, the alternator should be charging at a high current (the "Bulk Phase" referred to by CD). 4) After this preset voltage has been reached, the current starts to taper off - the "Absorption Phase", if you're interested.

The best way to ensure that batteries get the best charge they can, on the vehicle, and a very simple fix, is to change the alternator regulator for one with a higher output voltage. 5) Many of our original alternators will still have their 13.8V regs. Simply changing this for a 14.4V one will make a HUGE difference to how well your leisure battery charges. I have a fair bit of experience with boat charging systems, where there is usually a very large domestic battery bank, and the need to recharge as efficiently as possible. Much has been written about this subject by people who are real experts, and the consensus is that the best thing you can do 6) to ensure the fastest charge and the best chance of getting the state-of-charge closer to 100%, is to raise the charging voltage to just below the gassing point of the batteries. This will be 14.4 - 14.6V for the majority of lead acid batteries. The majority of boaters have no access to mains hook-up to externally charge their batteries, but manage a very efficient charge regime by using the alternator mods I've just described, combined with some solar charging for the final part of the Absorption Phase, which can often take a LONG TIME to complete, at much lower current.

One final thing to watch: try never to let your leisure battery to get below 50% state-of-charge (or about 12.2V under no load). Repeatedly pulling batteries down to a very low state-of-charge, where the lights go dim, etc, is a sure-fire way to destroy expensive batteries well before their time.

1) Although starting the engine can initially draw 120 - 160amps this is only for 2 - 3 seconds. The majority of the alternators work isn't replacing this 'initial loss' from the starter but keeping up with the current draw from the heated rear window (10 amps)/the blower motor (10amps)/the 20 amps from having the headlamps on full beam (square lights) etc etc As more electrics have been fitted the manufacturers have had to fit every bigger alternators to keep up with the extra current demand. Your typical car is now fitted with a 100 - 110ah alternator where as starter motors have not increased in power demand.
2) As I said, it is the PD (potential difference) between the regulated output of the alternator and the batteries voltage which in turn controls the degree of magnetism of the spinning rotor (stronger magnetic field equals higher current output & visa-versa)
3) Again as I said..output is not stepped, the alternator does not give FULL (BULK) output until terminal voltage equals regulated voltage....the output is infinitely variable from the alternators rated output (65ah) to practically nothing as terminal voltage reaches regulated voltage.
4) The output starts tailing off well before this point as I said ^^^^.
5) They never had 13.8 volt regulators ..Bosch /Beru original regulator spec for our vehicles was 13.9 - 14.2 volts.
6) Whilst I agree with your suggestion of fitting a higher output voltage regulator, are you saying that charging at a lesser rate does not produce gas? as my understanding is any charging produces gas but just at a lower rate.
Also....these higher output regulators (14.5 volts) were introduced in order to correctly charge newer calcium/silver batteries which require higher charge rates...these batteries are now the 'normal' and lead/acid car batteries are becoming a rarity.

Martin

[RedGus]

Martin, I apologise if you thought from the tone of what I wrote that I was disputing anything that you said, and I'm not trying to turn this into a p*ssing competition to prove who knows the most about alternators. My intention was just to add a bit to the debate on charging leisure batteries, based partly on personal experience, and partly on the evidence put forward by people who have spent years researching battery chemistry.

I completely agree with your point 1, but that doesn't help with the question of how to get charge back into a heavily discharged leisure battery as quickly as possible. If we assume that the alternator is adequately sized to cope with the running loads and provide spare capacity for battery charging, the thing that is most interesting for our purposes is: how do we charge a leisure battery as efficiently as possible?

2. Agreed. The regulator does nothing more complicated than compare the voltage at the output terminal (ie. close to that at the batteries) with an internal reference, and use this to infinitely vary the field current, and therefore the output current.

3. I think this contradicts what we agree on in point (2)! Surely the alternator is supplying full output, or thereabouts, almost all the way up to the regulated voltage? Then, the voltage remains capped at the regulator's preset value, and the charging current tapers off over time, until it is almost nothing (the Absorption Phase). It is this phase that can take a VERY long time, but this is the phase that ensure that batteries actually get fully charged, not just to 80% or so.

5. I didn't mean to make specific reference to the VW supplied alternators actually having a 13.8V alternator, but just to make the point that many older alternator regs will provide an output voltage which is too low for most efficient charging. This problem is made worse when you take into account the long cable runs from alternator to leisure batteries (and starter batteries in petrol models), which add a voltage drop, so the voltage the alternator "sees" at its output terminal is not the same as at the batteries.

6. The gassing point for lead acid batteries varies with temperature. At around 20deg C the gassing voltage is about 14.5V, and the optimum charging voltage is just below this point. Any voltage above about 12.9V will put some charge into a battery, and you CAN charge a battery at 13.8 - 14.2V, BUT these lower values will take AGES to fully charge the battery. The best compromise between length of charge time and excessive gassing would appear to be a point just below the gassing point for the type of battery being used.

The problem is always, as you hinted in your point (1), that these systems on vehicles were never designed to charge supplementary batteries, but just to cover the running loads of the vehicle and put a bit of charge back into a starter battery. Raising the charge voltage is the cheapest, simplest way of overcoming some of these problems, in order to charge leisure batteries alongside the standard vehicle systems.

Thanks for all your input, Martin.

[RedGus]

Thank you redgus interesting reading.
I have been banging on about traction batteries for a while now
I'd be interested if you have any opinions about them.

Thanks, Kev. I don't have any personal experience of traction batteries, but know that some people swear by them. This other forum (link below) is a useful place for advice on batteries and how best to charge them:

http://www.canalworld.net/forums/" onclick="window.open(this.href);return false;

Cheers,

James

[New Kentish Campers]

I now have a question for you ( and / anyone else listening) why is your split charge relay connected to your fridge at all?
The fridge should be connected only to the main battery , not the leisure battery , or its third in line to the power!

Steve, I've no idea to be honest. It's not been modified by me so I'm assuming its how Holdsworth converted it from day one.
All I knew was that it became warm to the touch and that I then found one of the fuse wires that come off from one of the terminals, to be in poor condition at the spade connector connection. Now that it's fixed all is well again. Indeed, I've just got home from a trip to Somerset and back and everything is good, no problems with warm wires.

And thank you too for the other later posts, sorry if I've made a bit debate out of it

Doug.

[Edited from reading Spare to SPADE connection]

[California Dreamin]

RedGus, it's all about the informatiion mate, never about the person, and definately never a contest...not for me anyway.
Regards as always
Martin

[RedGus]

Brilliant, that's what I thought!

Somewhere in all that information there might be something that can help a few people.

Cheers,

James