Battery-to-battery chargers

[Steve_N]

Quick question; do you have to specify a B2B charger of higher capacity than the alternator's output, or can you fit any size of B2B and just get a lower charge/transfer rate between the 2 batteries?

For example, can a 45A B2B charger be used safely with a system charged by a 90A alternator?

TIA.

 

[pvb]

For example, can a 45A B2B charger be used safely with a system charged by a 90A alternator?

Yes.

But there may be more effective ways of using your alternator's output, why do you specifically want a B2B charger?

 

[Steve_N]

why do you specifically want a B2B charger?

I'm afraid that I'm guilty of sneaking in a motorhome-related question Looking for the easiest way to improve charging of the leisure battery bank as the latest base vehicles seem to be supplying only 3-4 amps of the alternator's output to the converter's interface.

 

[Spi D]

If charging two separate banks from the engine alternator look at Swedish Septor.

Used for a ambulances, busses and delivery trucks w/tail lifts. Manages charge to two banks with priority on the starting bank.

www.tystor.com

No shares, just happy customer.

 

[pvb]

I'm afraid that I'm guilty of sneaking in a motorhome-related question Looking for the easiest way to improve charging of the leisure battery bank as the latest base vehicles seem to be supplying only 3-4 amps of the alternator's output to the converter's interface.

Ah, understand. Yes, probably makes sense for motorhome use. And no need to connect ignition feeds, etc.

 

[pvb]

If charging two separate banks from the engine alternator look at Swedish Septor.

That's essentially just a relay combiner. The B2B charger can boost the charge voltage and can have a multi-stage charging regime.

 

[Mistroma]

Deleted: Duplicate post

I messed up quote section & tried to edit.

PVB beat me to the punch while I was doing that.

 

[Mistroma]

If charging two separate banks from the engine alternator look at Swedish Septor.

Used for a ambulances, busses and delivery trucks w/tail lifts. Manages charge to two banks with priority on the starting bank.

www.tystor.com

No shares, just happy customer.

Sounds as if it is a relay system for splitting banks. So won't improve charging, just prevents engine start being discharged. Sounds similar to Smartbank system in that respect and that system doesn't really improve charging apart from giving priority to engine battery (if you wish to set it up that way).

I'll have another look at the website but only found reference to guarding the start battery.

 

[Stork_III]

I'm afraid that I'm guilty of sneaking in a motorhome-related question Looking for the easiest way to improve charging of the leisure battery bank as the latest base vehicles seem to be supplying only 3-4 amps of the alternator's output to the converter's interface.

On my MH, the batteries are connected via a 70A split charge relay, which connects batteries when engine running, independent of the converter's interface which itself isolates the habitation area electrics with the engine running, (a EU EMF requirement). The only supply available with engine running is to the fridge. Are you sure this is not what you are measuring at 3-4A at the interface?

 

[VicMallows]

On my MH, the batteries are connected via a 70A split charge relay, which connects batteries when engine running, independent of the converter's interface which itself isolates the habitation area electrics with the engine running, (a EU EMF requirement).

Having recently bought a caravan, I have struggled to understand the rational for the (non) 'habitation' relay which , as you say disconnects everything everything in the 'van other than the fridge when on tow. First time I have seen EMF (Electro Magnetic Field) mentioned as the reason. What is perceived to be causing an undesirable EMF, and what is it seen to be interfering with?

(I wire the boat as I choose, and will do the same with the 'van ).

 

[Stork_III]

Having recently bought a caravan, I have struggled to understand the rational for the (non) 'habitation' relay which , as you say disconnects everything everything in the 'van other than the fridge when on tow. First time I have seen EMF (Electro Magnetic Field) mentioned as the reason. What is perceived to be causing an undesirable EMF, and what is it seen to be interfering with?

(I wire the boat as I choose, and will do the same with the 'van ).

As I understand it, the requirement comes from a EU Directive to prevent disruptive EMF signals from the Habitation equipment affecting the vehicle electronic systems, and forms part of the Type Approval for the MH or caravan. Susequent owner modification are at your own discretion.

In the manual for my MH, it says.

"EMC Directive 89/336/EEC requires that electrical accessories within the vehicle are disconnected while the vehicle is in motion

 

[Steve_N]

On my MH, the batteries are connected via a 70A split charge relay, which connects batteries when engine running, independent of the converter's interface which itself isolates the habitation area electrics with the engine running, (a EU EMF requirement). The only supply available with engine running is to the fridge. Are you sure this is not what you are measuring at 3-4A at the interface?

I haven't gotten around to my own investigations yet but am rather going by the documentation that states that the leisure battery only charges at 3-4 amps, that I can't find any suitable heavy duty cabling coming 'aft' and that arriving home after a longish journey the batteries are some way off full charge. There's also a suggestion that the vehicle may have a 'smart' alternator that stops charging once the starter battery is recharged. Installing a B2B seems like it could be an easy way to get around these possible problems without having to delve any deeper or mess around with existing systems and wiring while still under warranty.

 

[VicMallows]

going by the documentation that states that the leisure battery only charges at 3-4 amps, that I can't find any suitable heavy duty cabling coming 'aft' and that arriving home after a longish journey the batteries are some way off full charge..

This suggests to me that the charge rate to the leisure battery is limited by the resistance of the (smallish) wires connecting it to the alternator. A B-B charger (at the leisure battery end) could indeed boost the voltage seen by the battery, and hence charge rate ..... but the current in the cabling back to the alternator would increase, resulting in even more resistive loss. The wiring could well become dangerously overloaded.

Ultimately, to achieve a higher charge rate at 12v you need heavier cable to the leisure battery. (leaving aside any ideas of converting the alternator output to a much higher voltage and then converting back down again at the leisure battery )

 

[VicS]

I haven't gotten around to my own investigations yet but am rather going by the documentation that states that the leisure battery only charges at 3-4 amps, that I can't find any suitable heavy duty cabling coming 'aft' and that arriving home after a longish journey the batteries are some way off full charge. There's also a suggestion that the vehicle may have a 'smart' alternator that stops charging once the starter battery is recharged. Installing a B2B seems like it could be an easy way to get around these possible problems without having to delve any deeper or mess around with existing systems and wiring while still under warranty.

The B2B is going to need its own heavy wiring from the vehicle battery and to the "domestic" battery in place of the existing. So maybe not such an easy get around.

However if it is under warranty and its not charging the domestic battery get it looked at before the warranty expires or you do something to invalidate it.

 

[William_H]

If we assume that the MH battery is being charged while the vehicle is in motion. (with (compressor) fridge running of the MH battery) Then the B2B charger will present to the MH battery a voltage in steps to charge the battery. The current into the battery will then depend on the charge state of the battery. Perhaps also the size of wires from B2B to MH battery.
The max capacity of the B2B will normally be way more than the battery actually takes.
Any resistance in the supply wire from the alternator to the B2B charger will not make that much difference because the B2B charger has a converter which takes almost any input voltage and produces the required output voltage. (by sucking ore current to make that higher voltage) ie maybe 13.5 v alternator out/ input voltage to 15v for fast charge of a low MH battery.
Now the B2b charger we imagine will not start to work until it senses that the alternator is charging like it has a VSR in its input. So a large voltage drop in the supply wiring could cause problems here but I imagine not likely.
In any case a reasonably heavy wire alternator to B2B would be desirable.
In answer to OP first question the B2B capacity will if low dictate the max charge that can be acheived into a large flat MH battery. The max size capacity of the B2B will not be limited in any way. Normally it will not acheive the max rating supplied current because the battery does not want that current. However in worse case big flat MH battery the large capcity B2B will suck a lot of current from the alternator resulting in alternator getting hot and its output voltage falling due to its charge capacity being exceeded. This might fall below the turn on voltage of the B2B resulting in alternator voltage rises to normal so B2B turning on doing the same thing.
A larger B2B is not likely to increase charge current unless the xisting B2B is very inadequate.
I would urge the OP to monitor voltage and charge current of the MH battery to confirm that the charge is in fact inadequate. and that that EU directive is not stuffing things up as we might expect it all to work.
Lastly most vehicle running is relatively long duration compared to a sail boat short engine run so charging is not normally a problem. The float voltage of the charging being the main limitation.
Regarding fridges. Many caravans have a 3 way fridge gas 240v or 12v. These suck a lot of current on 12v and are usually only run on 12v when in motion and gas or 240v AC when parked. This large current can diminish the performance of any charge system.
good luck olewill

 

[Steve_N]

Thanks for all the inputs. I'll try and cover some of the points.

There isn't a fault with the charging system, rather a deficiency, and I'd like to augment the charging provided by the converter. Clearly different converters will go to different lengths and some will have already provided equipment like VSRs or B2B chargers, but in the case of this particular budget converter it seems that they have done the bare minimum i.e used only that which is provided by the cab manufacturer at the converter interface, which isn't enough to charge the hab. batteries quickly and fully. Actually we don't have much of a problem and in the two three week holidays that we managed to fit in around our sailing last year it all worked just fine. Generally we're not big power users but when we do need to burn a lot of power, long dark cold Scottish winter nights for example, it's clear that we're returning home with less in the batteries than we started out with.

I would be running all new cabling of the correct size. The starter battery pod under the cab floor is only a short distance away from the hab. battery pod under-floor between the axles so this will be easy. There won't (see my plan next) be any increase in current drawn over the existing wiring.

Currently the two hab. batteries are selected by a 1-2-both switch as, when adding the second battery, I didn't want to overload the existing charging arrangements, either from the cab or via the onboard charger which is only 20A rated, so one battery is always isolated (from both charge and dis-charge simultaneously). I'm thinking that i'll connect the B2B charger to the isolated battery (using manual or auto switching yet to be devised..)., so the two charge systems will never be connected together and I won't even have to investigate the current wiring and relays etc. or run into any warranty problems.

So typically after a heavy overnight drain on one battery I'd switch in a fresh battery which will then be partly topped-up by the existing charge arrangements, and switch out the depleted battery for full charging by the B2B. Somewhat convoluted addmitedly, but far simpler than some of the systems on the boat!