Additional solar system & B2B for compressor fridge

[Deleted member 2045]

CAK tanks sell the shelf bars

 

[Markd]

I've 5m total run with 50A fuse.
I only used 100A relay because it had 6mm stud connectors.
Whilst one can get 9mm blade 70A relays cable terminals for 10mm2, space on the relay and direction of feed were the main issues for me.
I used soldered ring connectors which I could tighten up firmly.
I don't run an inverter off leisure (or anywhere!) So I don't really expect more than 20-30 amp unless I completely kill the LB.
10mm2 is 70A nominal and at that current the drop is 1.2v so current would start self limiting.
Added to which I've still got the original wire - probably 4mm2?
What also amazed me was the delivery speed of he relay from Hong Kong!

Update on my DIY B2B performance.
I've had the solar disconnected for a couple of days and have been using lights etc so a bit of power used on the LB.
So a good time to measure the voltage at the LB with and without the uprated Split Charge relay.
With the relay working I had 14.2v at the starter battery and 14.18 at the LB without the relay only 13.55v at LB.
So a definite improvement at less than £20 cost. Whilst I hadn't noticed any particular problem I can't have been getting a decent charge into the LB from the engine.
FWIW I had the fridge on DC while doing the test and turning it off made no difference - so the standard separate wiring for that works.

 

[in h]

I've 5m total run with 50A fuse.
I only used 100A relay because it had 6mm stud connectors.
Whilst one can get 9mm blade 70A relays cable terminals for 10mm2, space on the relay and direction of feed were the main issues for me.
I used soldered ring connectors which I could tighten up firmly.
I don't run an inverter off leisure (or anywhere!) So I don't really expect more than 20-30 amp unless I completely kill the LB.
10mm2 is 70A nominal and at that current the drop is 1.2v so current would start self limiting.
Added to which I've still got the original wire - probably 4mm2?
What also amazed me was the delivery speed of the relay from Hong Kong!

I really don't think this sounds like a specially good good plan. This is why:
If you are parked up for a few days, and drawing power from your hab bank, their voltage will drop to about 12v, but the starter battery will stay up around 13v. Start the engine and when the alternator cuts in, it will up the starter battery to nearer 14v.
Then the relay will cut in, joining the two batteries together through 5M of 14mm of cable, so the resistance of the cable will be around 7.5 milliohms, which means that 2v difference will try to push 300 amps into the hab bank.
Of course, the battery's internal resistance will cut that to maybe 100 amps, but it may still be way above the recommended maximum charge rate. It probably won't blow the batteries up, but it won't help them enjoy a long life.
That's the reason that motorhome manufacturers make the cable seem a bit thin: it's to limit peak current, not to save a few pence.

 

[Markd]

Interesting point - hopefully the 50A fuse I have in line at the hab battery would pop before any serious damage occurred and the existing wiring would allow safe recharging.
For the record my link cable is 'only' 10mm² but I take your point about it's ability to carry current.

 

[in h]

For the record my link cable is 'only' 10mm² but I take your point about it's ability to carry current.

I allowed for the other 4mm cable as well.

 

[Markd]

Oh I see what you mean.
The 4mm² cable from original split charge relay will probably protect itself by its higher resistance as you say that may be part of the design. It too is fused - 20-25A I think.
I think I'm content that by having a far better alternator charging system I'm less likely to get a severely depleted hab battery as it will be full when I park up and the solar should cope with consumption as we don't use a lot.

Thinking more about the whole current flow thing.
Isn't what happens with a heavily discharged LB just the same as jump starting a flat battery?
We don't particularly worry about melting the jump leads in the 'boost' phase but do normally wait before taking mega current for the starter.

 

[in h]

Oh I see what you mean.
The 4mm² cable from original split charge relay will probably protect itself by its higher resistance as you say that may be part of the design. It too is fused - 20-25A I think.
I think I'm content that by having a far better alternator charging system I'm less likely to get a severely depleted hab battery as it will be full when I park up and the solar should cope with consumption as we don't use a lot.

Thinking more about the whole current flow thing.
Isn't what happens with a heavily discharged LB just the same as jump starting a flat battery?
We don't particularly worry about melting the jump leads in the 'boost' phase but do normally wait before taking mega current for the starter.

There are three issues here.

Firstly it's the high current draw. Yes, we use jump leads, but there is a good reason why you're advised not to use leisure batteries to jump start. They're not designed to give (or take) high currents.

Secondly, it's running the hab battery down. Sorry, but your solution won't really charge the hab battery significantly higher than the standard, so it will run down under load when parked up with no sun.

Thirdly is the core issue and itself has a few aspects:

Your engine battery is a different types of battery, with more, thinner plates to cope with high currents in the starter battery and fewer, thicker plates to deliver power for a longer time in the leisure battery.

The batteries are at a different state of charge, but the alternator is set up to recharge the starter battery. That'll be recharged in maybe half an hour, and the alternator will switch to float voltage level long before the hab battery is even half way through the bulk phase.

To properly charge a battery, the charge cycle needs to have at least three phases, each of which is different according to how big the battery is and what its state of charge is. It is simply impossible for any sort of split charge or VSR system to do this.

There are three ways to charge the hab batteries properly.
1. You can fit a second alternator to charge the hab bank. That works, but few motorhome have the space under the bonnet.
2. You can fit a mains smart charger running off an inverter powered by the engine battery and alternator, but that's expensive and a lot to go wrong. Bit of a bodge, really, but if you have an inverter and a charger already, it makes some sense.
3. Most people choose the alternative of a battery to battery charger. Yes, they cost more, but they do the job really well. The unit properly charges the hab battery in a multistage cycle, respecting the chemistry, size and state of charge of the hab battery, leaving the alternator to look after the starter battery's needs. It works well.

 

[in h]

Of course, the wiring you've put in is ideal to fit a B2B charger to. They need at least 10mm cable.

 

[Markd]

If I had a genuine leisure battery I can see the point about it may be a different construction.
However it's a cheapo Platinum battery so is only really a rebadged starter battery.
In fact the vast majority of so called leisure batteries are starter batteries - just look at their CCA numbers which are typically getting on for 700A!!
They are often the same size as the starter battery and can be swapped straight in as replacements in the event of complete failure.
Thats probably why my motorhome builder (generically Swift) was (still is presumably?) quite happy to use the alternator to charge the battery via split charge relay - my low resistance feed replicates what a starter battery gets.
The point I made about delivered voltage via the van wiring is that for even moderate charge current they voltage drop is significant so recovery is slow.
It is my understanding that batteries have a considerable degree of 'self control' over the current they draw when charging.
I appreciate that ideally a battery should get multi phase charging but older motorhomes never got anything fancy from their Sargent PSUs.

I have a solar panel (as mentioned) so I don't expect the hab battery to have zero charge after an extended time unattended and when recharging the Votrobic regulator runs a 'proper' profile
Even when off grid for a few days I've not got the hab battery so low that it won't run my Truma heater which is pretty voltage sensitive so the panel is just about coping.
When I come to replace my hab battery (very soon I expect you'll say ) I will probably be going to a Yuasa EFB which is a genuine starter battery with very good cycling performance so should be able to withstand the 'abuse' I'll be giving it.

With B2B costing twice as much as a new battery I'm afraid that I won't be going there.

 

[in h]

Yes, Swift are indeed building down to a price. A starter battery in general is a very bad choice, because it will not have a good cycle life. Possibly 100 discharges to 50% if you are lucky.

Yuasa EFB are some sort of battery designed for stop-start systems, I think. Are they AGM? I'm not sure: Yuasa do such a big range of batteries that I lost count!

I have a feeling that they claim a 220 cycle life, provided you don't discharge below 50%.

Your 100Ah battery that can be charged to 85% full will have a usable capacity of 35Ah. But only about 250 times.

I can assure you that if you run a battery down so far that a Truma will switch off, it is more like 10% remaining, not 50%.

Yes, a B2B will cost more than a battery. So did lots of things in your van. It will do the job properly for many, many years, extend the life of your batteries and save a fortune in the long run.

Or you could spend money every couple of years replacing unsuitable batteries that your charging system has failed to cherish.

 

[Markd]

EFB stands for Enhanced Flooded Battery so it's a proper battery rather than AGM which aren't really suitable for leisure use.

12v Yuasa 100ah EFB Leisure Battery (L36-EFB) - Alpha Batteries

New exciting EFB technology. Ideal for Motorhomes, Caravans and Boats

www.alpha-batteries.co.uk

Another good alternative is the power frame technology offered by Varta/Bosch which have good cyclic performance and are high current batteries.

As you say taking any battery very deep will seriously reduce its total cycle life.
Having a B2B won't make any difference to that.
Ultimately the biggest factor in battery life is how you use it not how you charge it.

 

[SquirrellCook]

I suspect many battery manufactures have more than one label for the same battery. Even the name on it may not be who made it. Even within a manufactured batch I suspect the performance will vary greatly.

 

[in h]

EFB stands for Enhanced Flooded Battery so it's a proper battery rather than AGM which aren't really suitable for leisure use.
...
Having a B2B won't make any difference to that.
Ultimately the biggest factor in battery life is how you use it not how you charge it.

Oh, having a B2B will make a big difference. When you have a battery that you can only discharge to 50%, starting at 100% is very different to starting at 85%.
I think that AGM is very suitable for leisure use. In fact my batteries (which are proper deep cycle batteries, not leisure) are AGM.
I agree about Varta being a good buy.

 

[Markd]

I suspect many battery manufactures have more than one label for the same battery. Even the name on it may not be who made it. Even within a manufactured batch I suspect the performance will vary greatly.

Oh yes - battery megastore are near me and the import Hankook batteries but label them as all sorts for themselves and other retailers.
They sent me an unlabeled one by mistake recently but assured me that it was Hankook and had full warranty (needless to say I confirmed that back in writing just in case)

 

[wildebus]

....
Even when off grid for a few days I've not got the hab battery so low that it won't run my Truma heater which is pretty voltage sensitive so the panel is just about coping.....

Just on the point of the heater, I am not very familiar with the Truma, but the Diesel Heaters have a similar issue in that they will not start up if the voltage is below a certain voltage, and this can easily happen with the load from the Glow Plug (around 12A) and the thin wiring often used.
The cheap chinese diesel heaters are especially prone to this as they tend to underread the real voltage and are supplied with a very long and thin power loom - so even when batteries are not close to a worrying level the heaters report a low voltage error. Putting in a voltage boost regulator can be a very handy thing to do to get round this and save having to do the usual "fix" of starting the engine to boost the battery voltage. I have my heater on a 3-way switch so if it were to complain about a voltage error, I can flick the switch over to engage the boost regulator.

The same setup could work well with other heaters such as a Truma I would think?

 

[Markd]

Oh, having a B2B will make a big difference. When you have a battery that you can only discharge to 50%, starting at 100% is very different to starting at 85%.
I think that AGM is very suitable for leisure use. In fact my batteries (which are proper deep cycle batteries, not leisure) are AGM.
I agree about Varta being a good buy.

I'm not convinced that as I have MPPT solar keeping the battery topped up before any trip begins that I'm going to be starting at 85% charge.
But your point is valid and if I didn't have it I'd want to make sure the alternator got the best chance it could before I arrived at my destination and during moves.

My existing battery is 4 years old now and is still going ok.
We don't use a lot of juice so haven't felt the need to replace it or increase the number of batteries.
£20 didn't seem like a lot to spend and that included a proper earth for the hab battery - I hate to think what the alternator voltage at hab was when relying on the factory return through psu!!
Certainly if one does go up to 2 or more expensive batteries then most on board systems won't cope and 3rd party chargers will be needed.

 

[Markd]

@wildebus
The Truma 6002 will fire ok, then when the heat exchanger warms up the heater fan needs to speed up to deliver 6kw of space heating and that drops the voltage.
Once warm like the diesel heaters much less current is needed as the heater backs off to 2kw and runs slower

 

[wildebus]

Relay or B2B? there is no one size fits all solution.

Large B2Bs are expensive - and if you go the B2B and you want to get a good amount of charge current into a big battery bank that is fairly depleted you need to get a charger that is going to cost >£500. A lot more than a relay costing a lot less than a tenth of that and will do pretty much the same job at THAT stage of charging.
The B2B chargers come into their own when a battery is at around 80% of SoC or more - at which time the charge current is a lot lower and that big B2B will be putting out the same current as the smaller ones at half the price. And when a battery is at that level of charge, the basic alternator output just fed via relay will not properly finish the charging.

If you have a typical battery bank of around 150Ah or less, a B2B is the best overall solution, but if talking bigger banks (over 250Ah say), the best and most cost effective solution for battery charging from driving (without going into replacing vehicle alternators) is to use both!
Use a Relay to provide higher-current 'uncontrolled' charging, feeding as much current as the batteries will suck up at the lower State of Charge, and then switch to a B2B to finish the job when the current taken by the battery is within the realm of the B2B spec and the Battery SoC is at the 80% mark or so.
The CTEK 250S with SmartPass, which I know some members here run, works in this kind of manner. The basic CTEK 250S is a 20A B2B Charger, and the SmartPass 100A Add-on Module is just a relay packaged into a similar enclosure with some logic to switch the charging path between the two as appropriate. It is not a cheap solution though!
For my own setup, I currently have a high-current (230A) Victron VSR with 35mm2 cable to the starter (minimise voltage drop) and at the appropriate charging stage, a 30A Ablemail B2B charger cuts in and takes over. Sadly, my Alternator is not quite up to the job so cannot really exploit the setup I have until I repair/replace it!

 

[wildebus]

@wildebus
The Truma 6002 will fire ok, then when the heat exchanger warms up the heater fan needs to speed up to deliver 6kw of space heating and that drops the voltage.
Once warm like the diesel heaters much less current is needed.

Sounds like the same idea might work well with the Truma, just at a different point in the startup heat cycle?

 

[in h]

@wildebus
The Truma 6002 will fire ok, then when the heat exchanger warms up the heater fan needs to speed up to deliver 6kw of space heating and that drops the voltage.
Once warm like the diesel heaters much less current is needed as the heater backs off to 2kw and runs slower

A Truma 6002 has a maximum power draw of about 6A, but it is very rarely that high for more than a few minutes. A diesel heater needs about 12A for several minutes as it starts up.
Trumas are supposed to be OK as long as the supply voltage is more than 10.4v. I doubt many systems drop that low with a 6A load.