Voltage drops across the entire system

[MidLifeCrisis]

Anyone like to comment on this option ....

It occurs to me that the issue here is that we have two batteries linked in parallel;
- one (the leisure battery) which is potentially quite drained (from a night of lights, radio, dvds, propex use etc)
- and the other (the starter battery) which has done a short amount of work starting the engine

With the engine running, the alternator 'sees' a voltage which is a 'combination' of the two batteries and regulates the voltage coming from the alternator accordingly.

So there may be an initial 'surge' of high current as the two batteries are charged up but as the starter is pretty 'full' already the 'combined' voltage quickly rises to a point that the alternator is barely doing anything. While the leisure is actually still pretty drained.

.... So, it would seem that the ideal solution might be to 'remove' the starter battery from the system once it has achieved a decent charge level. This would leave the alternator only 'seeing' the leisure battery voltage and therefore providing a decent charge level to the leisure battery to quickly charge it up?

Is this an option?
Would it work?
Is it wise?
Thoughts?

[CJH]

.... So, it would seem that the ideal solution might be to 'remove' the starter battery from the system once it has achieved a decent charge level. This would leave the alternator only 'seeing' the leisure battery voltage and therefore providing a decent charge level to the leisure battery to quickly charge it up?

Is this an option?
Would it work?
Is it wise?
Thoughts?

How much current does a running engine draw? If your starter battery was disconnected form the charging circuit, how long would it be before the starter battery started to get seriously depleted? I have no idea, except that even during daylight running, my dashboard voltmeter tells me that at high revs the engine is pulling more from the electrical system than at idle (because more sparks are needed I assume) - I see a drop in the voltage. With the lights and/or blower the draw would be higher. How would such a system decide when to switch the starter battery back in?

[MidLifeCrisis]

How much current does a running engine draw? If your starter battery was disconnected form the charging circuit, how long would it be before the starter battery started to get seriously depleted? I have no idea, except that even during daylight running, my dashboard voltmeter tells me that at high revs the engine is pulling more from the electrical system than at idle (because more sparks are needed I assume) - I see a drop in the voltage. With the lights and/or blower the draw would be higher. How would such a system decide when to switch the starter battery back in?

I'm suggesting removing the starter battery completely from the system so the van would be running off the alternator and leisure battery - i.e. keep the starter battery for exactly what it says it is - to 'start' the engine, then let it charge back up, then remove it from the circuits

[CJH]

Got it. I misunderstood

[MidLifeCrisis]

Got it. I misunderstood

No probs - so what do you think - is it silly/unnecessary/too complicated???
It's sort of exceeding my knowledge a little bit so I'm nervous to try it out? And think I'd need a good high amp ammeter to measure to see if it was doing anything??

(I know that a Sterling would achieve the same via a slightly different but less invasive method .... but at a cost >£300!!!! that I don't think I can justify ... and can't afford even if I could!!)

[marlinowner]

Surely when the two batteries are connected together by the charging relay, charge will flow between the batteries until they are at equal voltages. The alternator will charge them both equally.

[itchyfeet]

Ghosts poor mans Sterling is a smart battery charger and an inverter surely you could do this for £100 if you wanted to charge the leisure quicker.

[lloydy]

Something like this will do it
http://www.seamarknunn.com/acatalog/ste ... IBvvWIgGSM" onclick="window.open(this.href);return false;

[MidLifeCrisis]

Surely when the two batteries are connected together by the charging relay, charge will flow between the batteries until they are at equal voltages. The alternator will charge them both equally.

The way I understand it (and hopefully someone smarter can confirm) the batteries are in parallel so the overall voltage 'seen' by the regulator is an average of the two voltages - as the alternator charges the batteries it will provide less and less 'power' as that combined voltage is raised up; so the lower voltage battery will never adequately charge (or will take forever to do it ...... i.e. many hours) ....... so not very efficient.

Ghosts poor mans Sterling is a smart battery charger and an inverter surely you could do this for £100 if you wanted to charge the leisure quicker.

Agreed - Ghosts idea is a good way to get that higher voltage to the leisure batt - but was just exploring if there were other options available

Something like this will do it
http://www.seamarknunn.com/acatalog/ste ... IBvvWIgGSM" onclick="window.open(this.href);return false;

As far as I can tell this takes over control of the voltage regulator function from the standard regulator in the alternator ..... But it does involve hacking wires into the regulator which might be an option if indeed that is possible (i.e. you can tap into the regulator somehow)?

[bigherb]

How much current does a running engine draw?

About 8-12 amps depending on engine with carburettors, double that for fuel injection.

Surely when the two batteries are connected together by the charging relay, charge will flow between the batteries until they are at equal voltages. The alternator will charge them both equally.

That's pretty much it, and a well executed split charge relay system will work perfectly well at reasonable cost and simplicity of installation.
Leisure battery use has many variables though, depending on how you use them. Someone who uses the battery moderately and and uses the vehicle to tour when camping will keep the battery charged adequately or hard use and then uses the vehicle as a daily driver after camping will replenish the battery in good time, if you lay the vehicle up after camping then the battery will need a full charge from an external source, solar or main power. If you use mains hook up with a charger or adequate solar charger it matters not, the battery will normally be kept topped up.
The vehicle charging system will bulk charge the battery to around 80% charged fairly quickly but the last part of the charge takes considerably longer whether it is direct charged from the alternator or through a split charge relay.
What people are trying to do here is decrease the time it takes to push in the final charge into the battery by raising the charging voltage, but it has to be managed to keep it below the gassing voltage of the battery which decreases the hotter the battery is, hence keeping the battery cooler out of the of the engine bay will allow us to charge the battery quicker at higher voltages. For the vast majority a split charge relay system will suffice preferably keeping the leisure battery charged at 13.8V-14.2V. For some it may not and then it becomes how cost effective it is getting a marginal decrease in charging time.

[irish.david]

Hi,

It’s certainly an interesting alternative for charging a leisure battery, but it’s only really suited for situations where you don’t let your leisure battery get too low. As the charger is limited to supplying 10 amps then it’ll take much longer to get an empty battery to complete the bulk charge stage (about 80% full as bigherb has mentioned) than a normal split charge system in good condition.
Another problem would be that if there are any high current items on the leisure circuit that are in use when driving then the battery could actually discharge with the engine running if the load was over 10 Amps, so this would rule out things like 3 way fridges running on 12v.

As i’m sure you’re aware, the main problem seems to be in the connection between the starter battery and the leisure battery so if you replaced those with upgraded cabling and a brand new relay then you’d notice a huge difference. For about £15 you could get 3m of 16mm2 cable and a 70A relay and with those the voltage drop from starter battery would only be about 0.3V (Assuming the starter battery voltage is 14V, the cable length is 3m and the charging current is 50A).

Another big improvement would be to replace the cable running from the starter motor to the battery and also the cable from the alternator to the starter motor. Using 60mm2 cable for the battery to starter motor and 16mm2 cable from the alternator to the starter motor you should be able to get the overall voltage drop from the alternator to the starter battery at full charging current (90A) to about 0.4V (Assuming alternator output is 14.4V, and the cable lengths are 1m for the 16mm2 and 4m for the 60mm2). This also has the advantage of the starter motor being supplied with much more power for starting so the engine should start quicker.

Years ago i was considering running an extra cable between the alternator and starter battery and connecting the batter sense wire directly to the battery terminal. In theory this would mean that the alternator would supply whatever output voltage that would be required to produce 14.4V at the starter battery terminals. Problem would be that the engine electrics are connected directly to the alternator output so you’d have to add a 12V regulator to protect them, but in theory that could have worked. In the end I found that by replacing and upgrading all the high current carrying wires that the voltage drop wasn’t such an issue.

Dave

[CovKid]

I'm with you Dave. All these figures are academic, at least for me. There are other factors that will also come into play such as what you use when parked. I use a very simple relay and everything charges fine. I'm using the propex a lot now too but had no issues. I upgraded all the main cables this year to chunky stuff with assistance from 1664 (thanks Bren) - even up to the fusebox and whilst its not a perfect analogy, bigger pipes move more water. Thats the rationale behind it anyway.

Apart from ignition for Propex, I buy and run very low consumption stuff. If you're not sucking power all the time, even a modest charge should keep the old power banks in good order, voltage drop or not. However, upgrading main cables makes quite a difference. I gained around half a volt from that change alone. You can end up chasing your own tail trying to get a completely lossless system otherwise.

Neighbours must think I've flipped my lid - been out three times tonight to play with those new LED interior strips.

[ghost123uk]

I'm with you Dave.

I am with Dave too really.

I was / am just playing about (as mentioned in the OP). It's not that I have a problem, I just like to experiment and having a "smart charger" permanently feeding the leisure seemed worth a play. It works well, so for now it can stay. Next year I might crawl around underneath and fit better cables \and connectors as Ralph has done to reduce current induced voltage drop.

I might also experiment with testing just how much current, and how long for, is taken by a 50% depleted leisure battery on initial start up when connected to the starter via the more usual split charge relay. I mentioned further up that I "think" it may be not a high value for very long. Reason being that the voltage at the leisure battery may be artificially "seen" as higher than it is, due to it being in parallel with a well charged starter battery (the "averaging of voltages" thing mentioned by someone else above). I could be wrong of course

Dave, you mention =

Years ago i was considering running an extra cable between the alternator and starter battery and connecting the batter sense wire directly to the battery terminal. In theory this would mean that the alternator would supply whatever output voltage that would be required to produce 14.4V at the starter battery terminals. Problem would be that the engine electrics are connected directly to the alternator output so you’d have to add a 12V regulator to protect

You would not need a 12 Volt regulator to protect anything on the vehicle from 14.4 Volts. Everything on our vans is just fine at 14.4 Volts (even car stereos and alarms etc are fine up to 16 Volts)

Oh, and re running "stuff" off the leisure battery whilst charging. In my case this won't be a problem as, when driving, the fridge runs off the starter battery (and therefore the alternator) as it should and the only other load off the leisure battery is the (fairly modest) music system.

[CovKid]

There is some variation between alternators as we know (or at least the regulators) but for instance, I have a 90 amp alternator - lots don't. That too may have a part to play. I'm not saying a 90 amp is better or will have any effect on voltage drop, but it may well improve charging under some situations.

Voltage drops aside, I'm not doing what might be deemed an acceptable running time to charge both batts - I perhaps do six miles a day (3 miles each way) in my working week and I've got the lights and propex on both ways but perhaps the 90 amp copes better with that. Could be.

N.B. Holy grail for me would be a 120amp alternator but they didn't make them in that form factor so it would need modified brackets made up (and some rewiring) to make it work with a different one but that kind of power seems more in step with a camper that has two batteries being charged - and, I hasten to add, more readily available.

[ghost123uk]

Aye, if "we" had an Amp meter with a high enough rating (a bit rare, needing to be one of those electronic "shunt" types) it would be interesting to watch the current levels (of both batteries) during the various periods of charging off the alternator.

E D I T = or if we could accurately measure the resistance from alternator to starter cable(s) and use the voltage drop, we could work it out that way (the cable acting as the shunt), but not many meters have an accurate enough resistance scale (my Avo 8 has ) or maybe someone has a Wheatstone bridge