Exceeding ‘recommended’ Battery capacity?

[Heebson]

Hey folks,

I’m looking to swap out my 97Ah lead acid battery for lithium.

Context : it’s only a 06/19 registered van, but battery seems goosed. When we first viewed the van before buying the battery was completely flat (and had been sold to dealer 5 months prior). It was charged on second viewing, then flat on collection. We were off-grid recently and even with starting on full charge at home, then being out of the van most of the time and very light usage (water pump, occasional heat blower and charging two phones) plus some driving - it dropped below 10v abruptly after not quite two days. So, I reckon that’s at least 3 full discharges, probably more. We do intend to spend more time off-grid.

I have very limited space in my Nuevo, so probably talking about a single battery drop-in replacement. I’d obviously like to maximise capacity (within financial sanity).

We have an Sargent EC700 Power Control System (and PX300 charger) and the manual has this to say :

“For optimum performance and safety it is essential that only a proprietary brand LEISURE battery is used and it is suggested to select a battery from the NCC Verified Battery Scheme with a typical capacity of 75 to 120 Ah (Ampere / hours).” & “The PSU is configured to work with standard lead acid leisure batteries, and in most cases is also compatible with the latest range of Absorbed Glass Matt (AGM) batteries. The system is also suitable for Lithium batteries with built-in Battery Management Systems BMS).”

What I’m really not clear on is the implications if I drop in a battery greater than 120 Ah?

There’s also a section on Sargent help site about whether a B2B (or DC-DC as they call it) is needed, but all I’m seeing is an explanation of what it is, rather than a way to decide! (https://sargent.zohodesk.eu/portal/en/kb/articles/do-i-need-to-install-a-dc-dc-charger). Reading around, the implication seems to be that B2B might be required for older vans - so can I make the assumption that I won’t need this?

Many thanks in advance for any advice.

Heeb.

 

[autorouter]

First, yes it does sound like the 97Ah battery has had it. If a battery is left in a discharged state for weeks or months then sulfation occurs, and the battery will seem to charge and discharge OK, but will behave as if it has half the capacity or less.

A standard split charge relay connects the starter and leisure batteries together when the engine is running, so that the alternator can charge both as if they were one big battery. That works fine if the two batteries are the same type. If the leisure battery is different, then is still works OK, but not optimally.

Some people find that is satisfactory for what they need, others are disappointed that the battery doesn't charge faster. One of the benefits of lithium is that it can charge much faster than lead-acid types.

Another problem with lithiums is that they shouldn't be charged when it's freezing or below. A good BMS built into the battery will prevent that.

If you want faster, more accurately tailored charging, then a B2B will give you this. It will also prevent charging at low temperature, instead of relying on the BMS.

I'm presuming your van doesn't have a smart alternator, but if it did then a B2B is necessary because a standard split charge relay will only partially charge a leisure battery.

 

[Jezter]

".... occasional heat blower..." - bit worried this might be a fan heater or similar, which might have implications for new installations or the present one? Without wishing to teach you to suck eggs, you might want to check the power requirement on this.

 

[rwg]

I suspect that the 'heat blower' is using the battery to run the gas heating system fan (Whale/Truma) - that shouldn't be an excessive load as it's just moving the air around, not doing the heating.

As to what happens if you put in a bigger capacity... Interesting question. I think anything bad will be limited to the 'much bigger' category, not the 'just a bit bigger'...

I suppose a much bigger battery might draw a lot of current when charging from nearly flat, and that could be more current than the system was designed for. If you are charging on split charge while running the engine, maybe it could blow the fuse at the battery if say the fuse is 20A but the current to the battery hits 30A. On hook up the Sargent PSU looks to top out at 25A, so charging rate will be limited by that limit on available current.

Maybe the PSU would get hot if it had to run at full power for several hours to charge a larger battery from flat, but modern power supplies should protect themselves from over current/over temperature etc.

cheers,

Robin

 

[pappajohn]

Fitting a larger battery will simply mean it will take the charger longer to fully charge said battery.
The battery charger will supply as much as it's rated at, for example, a 20a charger will provide power for up to a 20a load.
Anything over 20a and the difference needed is taken from the battery to cover the extra amps needed.

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[autorouter]

I think some chargers like CTEK and Victron are designed to be used at full power continuously 24/7 if necessary, so they can be connected to a battery of any capacity and just continue to charge until it is full, even if it takes a week. I've had a 5A CTEK charger charge 400Ah of lead-acid batteries in the past, and never overheated or had any other problem, just did the job. However I think a lot of the built-in chargers would overheat and possibly fail if asked to do the same.

If you go for more than 120Ah you may need to think of changing the mains charger. But if you are switching to Lithium it's a good idea anyway, you can get one with a lithium charging profile. and maybe even a temperature sensor. However if you are rarely on hookup you'll probably get away with the standard built-in charger.

 

[Heebson]

Thanks for the replies.
I guess at this stage, I’m hoping to be least invasive. Reading the documentation, I know I’ve a split-charger and clearly the leisure battery was being charged by the engine (other than that, I’ve no way I can tell if it’s a smart alternator).
Is the B2B charger something that can be straightforward to install at the leisure battery side? I’ve no desire to be popping trims to get new cabling running from the engine.
The batteries I’m looking at all seem to have heaters as well as BMS.
Think it’ll be a 120Ah drop in, as that should more than double what I’ve been getting to date.
Oh - and yes, the ‘heater’ was referencing the air blower on the Truma powered by gas off-grid.
Thnx,
Heeb

 

[autorouter]

I think if the existing battery is being charged OK by the alternator with the split charge relay, then you don't have a smart alternator.

Fitting a B2B is reasonably simple but not pain-free. Depending on the amp output of the B2B you may need to install new wiring from the starter battery to the B2B,and from the B2B to the leisure battery.

If the B2B is 30A or less, the existing wiring could be used, and it would have to be wired across the starter battery and leisure battery connections of the EC700. If it's more than 30A, say 60A, then new wiring to both batteries will be required. One of the main reasons for fitting a B2B is to increase the charging amps into the lithium battery.

In both cases, the existing split charge relay built into the EC700 will have to be disabled. I don't know exactly how to do that. There will be a way to do it, but it usually depends on the details of the specific unit. Maybe there's a menu setting, maybe a jumper link can be removed, or something similar.

The heater in the battery is for use in freezing conditions. They can't be charged while at freezing or below. So the choice is to wait until the temperature rises, or to divert the charging amps to the heater until they are warm enough to take a charge. That means there needs to be some kind of temperature sensing that controls the charging and heating.

 

[Hoovie]

I think if the existing battery is being charged OK by the alternator with the split charge relay, then you don't have a smart alternator.

Fitting a B2B is reasonably simple but not pain-free. Depending on the amp output of the B2B you may need to install new wiring from the starter battery to the B2B,and from the B2B to the leisure battery.

If the B2B is 30A or less, the existing wiring could be used, and it would have to be wired across the starter battery and leisure battery connections of the EC700. If it's more than 30A, say 60A, then new wiring to both batteries will be required. One of the main reasons for fitting a B2B is to increase the charging amps into the lithium battery.

In both cases, the existing split charge relay built into the EC700 will have to be disabled. I don't know exactly how to do that. There will be a way to do it, but it usually depends on the details of the specific unit. Maybe there's a menu setting, maybe a jumper link can be removed, or something similar.

The heater in the battery is for use in freezing conditions. They can't be charged while at freezing or below. So the choice is to wait until the temperature rises, or to divert the charging amps to the heater until they are warm enough to take a charge. That means there needs to be some kind of temperature sensing that controls the charging and heating.

There won't be any way to turn off the split-charge function in the sargent unit in terms of selectable options.
Methods on how to do this have been discussed in various threads, and how it is done can affect other functions.
e.g. Remove the D+ signal going in. The Sargent then doesn't know the engine is running so the Split Charge won't work. However possible knock-on effect is Hab Electrics don't disable (bonus result I reckon!) and also possibly 12V to Fridge no longer works and Electric Step doesn't self-retract on engine start. These last two may or may not happen depending on the model of Sargent PDU (on my EC325, the 12V Fridge and Step are controlled by D+ but in an EM50 module so unaffected by removing the D+ into the main unit).
I think a neat option is to fit a 5-pin relay in series with the Starter V+ into the Sargent, using the 87a NC Pin to the Sargent and controlled by the D+, so when the engine starts, the D+ turns the relay on and removes SB V+ and the Split Charge is therefore inoperational.
This could be easily done as part of the B2B install when the wiring is adapted for that.
Some people do open up the sargent box and snip wires or remove relays - personally I reckon that is a bad idea when there are ways to achieve the same result with methods 'outside the box' (literally)

 

[rwg]

My approach to disabling the split charge built into the Sargent (EC176 in my case, but I would expect this to work across the board) was as follows.

* Deliberatly blow a spare fuse
* Solder a 1K resistor across the pins of the blown fuse
* Replace the fuse marked 'Vehicle Battery' in the Sargent ECU with your new 'fusistor'

The Sargent unit still knows the engine is running and the fridge/step work fine (at least mine do). The hab electrics still turn off when you start the engine. The vehicle battery voltage even shows up on the display panel.

However, the split charge circuit cannot charge the leisure battery (or discharge it back into the vehicle battery if you have a smart alternator), since the 1K resistor limits the current to a few milliamps.

Downside is that you cannot run the hab electrics from the vehicle battery unless you replace the original fuse, but you would be unlikely to actually do that except in an emergency anyway.

If you want a simpler solution, just pull the 'Vehicle battery' fuse in the ECU, but then the control panel will show 'Vehicle battery disconnected' or similar.

cheers,

Robin

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