Alternator to leisure battery whilst driving

[costaexpress]

From a previous thread I opened I realise, from the excellent contributions, that my ideal solution is a b2b with uprated cabling, however, for a number of reasons I have not been able to have this installed just yet and with an impending trip involving off grid overnights I was just wondering what, if any, charge I can expect to the leisure batteries when driving. I have 2 x 105AH batteries and a standard Ducato cab (alternator). I also have a compressor fridge which although it only averages 2 amp can draw 5 amp when the compressor is running. My understanding is that once the cab battery is charged the alternator almost stops pushing any power to the leisure batteries, therefore if the fridge is drawing 5 amp at times and the cab battery is fully charged will my battery bank actually go down whilst driving rather than be recharged. Hope my comments make sense.

[mikefitz]

I dont think your van has start stop and smart alternator so any issues with that type of system can be disregarded.

 

The standard system alternator generates somewhere between 14.2 and 14.4 volts, seen at the engine battery. This remains more or less constant at the engine battery when the engine is running. If the engine battery were heavily discharged, the voltage may be somewhat lower initially in the high 13v region, but would soon reach over 14 volts and remain at that level whilst the engine is running.

 

When the engine is running, the electrical control system will allow a connection between the engine and leisure battery, allowing the leisure batteries to charge if required, and also supply any loads on the leisure battery. Even if the interconnecting circuit is less than ideal, there should be no drain down of the leisure batteries, as the alternator voltage of over 14 volts has more 'push' and will supply the fridge load.

 

The compressor fridge, when on, will take 4 to 5 amps 'extra' from the alternator, that is well within its capacity.

 

With a typical fridge duty cycle, the average current will result is a use of around 40 to 50 Ah per 24 hours.

 

When parked up off grid, assuming your two batteries are in good condition and fully charged, you have almost two full days without undue stress on the batteries, providing other electrical loads are modest.

 

Mike

 

 

[Brian Kirby]

Do remember, however, that it is more than a case of alternator output, because the rate at which the leisure batteries has also to be taken into account. If you are able to connect you van to mains for 24 hours or so before you set off, so that you are starting with fully charged leisure batteries, by the time you get to your destination they should still be fully charged.

 

But, if you start with the leisure batteries partially discharged, you may well not arrive with them fully charged, and will have some difficulty assessing their actual state of charge on arrival.

 

To eliminate that uncertainty you need to know their remaining capacity when you leave, in Ah, because, even though that is nominally 2x125Ah, their age and previous treatment (assuming they were originally a matched pair), will have reduced their actual working capacity. You then need to know the rate of charge, in Amps per hour, that the alternator is supplying at the batteries, allowing for re-charging the starter battery, running the fridge, and any other possible leisure battery loads.

 

Then you will know how much needs to be put in, and the rate at which the batteries will be re-charged, so can assess how much driving time is necessary to fully re-charge the batteries. For the former you need to fully charge the batteries, rest them for (say) 12 hours, and then check their voltages. That should indicate their current maximum capacity as a percentage of their capacity when new. You then need to test the batteries by connecting a known load, and then monitor the rate at which the voltage reduces over time, ceasing when they reach a 60% discharged state. Use Mike's figures for the average load form the fridge, unless you have figures for the actual fridge in your van. Any other connected loads will probably be low enough to discount, but you'll need to check that this is the case. You then need to know what the usable alternator output is in Amps, and how long it is likely to take you to reach your destination.

 

If you can give the year and model of your base Ducato, someone should be able to advise the reckonable alternator output. And, if unclear how to assess the current capacity of your fully charged leisure batteries, and how to test their ability to deliver that capacity over time, I'm pretty sure others will be able to advise on these as well. From what you have said about your motorhoming preferences, I imagine you will probably find the resulting answers to the calculations of great reference use in future, so presumably well worth the faff!

[Deneb]

I have a van with a compressor fridge that has very similar consumption to yours, and was delivered with a simple split charge relay between the vehicle and leisure batteries (2 x 90Ah). The van will see all three batteries as a single battery when they are connected by the relay, so will continue charging until it sees that the "battery" is full, unless of course your journey ends before that. If you are driving several hours each day between overnight stops, you may well find that the leisure batteries are fully charged daily, but even if not moving at all, I found two to three nights easily achievable with that setup (my van also had a 100W solar panel ,but you can largely discount any effects from that at certain times of year, and probably in the current weather conditions.

 

We did manage five consecutive nights off grid on a November week in Yorkshire a couple of years ago without moving the van or starting the engine at all, and still managed to keep the leisure batteries above 50% SOC. If you want to maximise your ability to survive off-grid without any additional equipment, there are other tricks you can employ. We carry several ice-packs in the freezer compartment of our fridge during the day, and distribute them within the main fridge compartment overnight, whilst additionally either turning the fridge thermostat down or off altogether if the ambient temperatures are fairly cold, such as at present.

 

I regard a battery monitor such as the Victron BMV or NASA gauges as essential, as when properly calibrated to your van they give an almost instant indication of the state of your batteries, consumption, and how much longer you can go before needing to recharge. Since fitting such a device in our van, I have found that 27Ah is a reasonable 24 hour average for our electrical consumption - somewhat less than Mike envisages, but that involves thinking about the electricity that you are using and minimising it as far as possible, e.g. we tend to limit television to an hour or so an evening, if at all, minimise the use of lighting and read from Kindle paperwhite's in the dark, use the ice-pack trick in the fridge, etc.

 

Within the last couple of years I have modified our van and now have 200W solar, an MPP regulator rather than the original PWM type, a B2B charger and of course the battery monitor. Last September/October we spent six weeks touring Scotland with only three nights on EHU, no problems at all. We only used the EHU on those three occasions because it was there, we didn't really need it, but we are happy to forego the use of items that some may regard as essential to pursue the enjoyment of overnighting off the beaten track. If you cannot survive without a coffee making machine, hair dryer etc. hours of watching TV or perhaps need to recharge electric bike batteries or medical equipment etc. things might well be different.

 

Bottom line though, I would expect that you will find a couple of night off-grid quite manageable as you are with a little forethought!

[costaexpress]

Thank you for some great feedback. I think the change of van and change from a 3 way fridge to a compressor fridge, has introduced a kind of range anxiety that I didn't previously have. When I think logically about it, I have had many nights without EHU so alternator must be replenishing the leisure batteries when I drive between sites. I have definitely been reading too many articles for my own good and I picked up somewhere along the line that the alternator virtually shuts down once the engine battery is charged (or at least that's how I interpreted it!) leading to my latest concern, however, I would not have survived to date if it was as simple as that. Brian's suggestion that I gain a much greater understanding of what is going on in the system is a good idea both for performance and my own piece of mind so I'll get on with that and continue to do in the van what I have always done and maybe introduce a few of the fridge management tips you've kindly provided.