# Battery to battery chargers - nonsense?



## pieterv (Feb 3, 2009)

Hi,

I was all convinced of the case for a battery to battery charger: the car battery needs to be charged with a lower voltage than the gel battery I have for the domestic side; car manufacturers set the alternator regulator to charge the car battery, so the gel battery never gets fully charged.

Almost set to buy one, but thought I would just measure the charging voltage (i.e. with engine running) and this turns out to be 14.4V. the exaxt voltage my gel battery manufacturer specifies (and a similar voltage to what battery to battery chargers provide)!

Utterly confused, I checked the current that the alternator put in the domestic battery on the control panel, switched the engine off and connected up to mains. The built in Schaudt control block with IUoU charger (i.e. optimised for domestic battery charging) put in the same current as the alternator.

So, what is the use of all these battery to battery chargers if the alternator already gives the correct voltage? What am I missing?

BTW checked the voltage on the car, which was roughçly the same, so it isn't the case that the regulator on the van is bust.

Pieter


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## Jean-Luc (Jul 21, 2005)

Hi Pieter, I fitted a Stirling B to B charger last spring and have seen the benefits. It will 'whack' in power at up to 50 amps to quickly recharge the leisure batteries after off EHU use which gives a very short time to total recovery. It will then float charge at 14 volts to give a fully charged battery. An example of this was when we stopped for 48 hours in La Rochell last December in sub zero conditions and had the heating on continuously plus generous use of lights, sat box and tv, at the end of the stopover we were still showing 12.2 volts. When we reached our next stop at Mayenne the batteries (2 X 95 amp) had fully recovered. BTW the B to B charger works best with semi-traction open wet batteries as the charging voltage can be set for 14.8volts for maximum charge BUT the electrolyte level must be checked regularly and topped up if necessary.
Colin


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## blondy (Aug 23, 2007)

I think it all depends on the system fitted to the M/H, some of the newer M/Hs appear to have a high output charger fitted, therefore no need for the BtoB charger, I have one fitted to mine, its been very successful.


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## Jean-Luc (Jul 21, 2005)

My MH is a 2005 Rapido and it does indeed have a op of the range 4 stage charging system with a max output of 16 amps BUT this is mains supplied and only operates when on EHU. The poor charge rate, which is deliberately low voltage to enable sealed no-maintenance batteries to be used in the base vehicle, and which comes through via the split charge system when the engine is running is what the B to B units address.
Colin


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## Snelly (Aug 20, 2005)

Volatge will be 14.4v... but what amperage was it getting?? It wont of been as much as 50 amps that the b2b chargers can put in.

They work by making the alternator continue to output more even when the engine battery is charged, in essence it fools the alternator into thinking the engine battery is low, when all along it is diverting the charge into your leisure battery until charged. Of course it makes sure the engine battery is fully charged first as a priority. It does all this intellegently, using stepped charging cycles, looking after your batteries condition.


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## JeanLuc (Jan 13, 2007)

Well I am not entirely convinced about the B2B charger. I was very close to having one fitted a few months back, then took the trouble to watch the charging currents in the van. It is a standard Hymer installation with a Schaudt EBL99. When on EHU, the charging current is 18 amps and voltage rises to 14.3V during the first stage of the cycle. Then voltage stays at 14.3 whilst current falls back as the batteries become charged during the second stage. Finally current drops right back and batteries are float charged at 13.8 amps in third stage. The duration of the second stage depends upon whether the EBL is set to Lead-Acid or Gel: the former in my case since I have Elecsols. (The technical adviser at Elecsol said this was a perfectly good charging regime for their batteries.)

So far so good, but what happens when the alternator is doing the charging? Immediately after starting the engine, the leisure charging current shown on the Schaudt control panel hovers around 7-10 amps. Then after a short time (which I presume is allowing the vehicle battery to get charge back) the current goes to something over 30 amps. I don't know exactly how high as the needle is on its maximum stop at 30A. It stays at 30 amps plus for some considerable time - probably a couple of hours at least.

Since I have heard of certain compatibility problems with B2B chargers and EBLs (Roadpro's catalogue refers to the battery status readouts not being accurate) I decided to leave well alone and rely on what looks to me like a pretty reasonable alternator charging current.

Am I missing something given my limited knowledge of automotive electronics?

Philip


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## NicknClair (May 18, 2006)

I have to say that having one fitted in my camper for the last 10 months, we both have been over the moon with it's performance and reliability.
Example was in August where we were only able to get a grass pitch as we left the booking too late, had the diesel heating on, using the inverter for various things (TV, kettle, Partner's Hair stuff etc etc) and took the 220AH Elescol down to 11.7V after roughly a couple of days. Had to move on to another site (roughly just over an hour away) and hey presto back up to 12.9V and fully charged. 
We are using ours in a way that we have a Battery Computer on board that as well as Volts and amps reading, it gives a time to charge and time to discharge, meaning we know how long to run the engine on idle to charge the leisure battery should we are not moving off anywhere.
Was going some work on the van recently and took the capacity down to around 80%, started the engine and looked at the amps in on idle (bearing in mind that our van has clocked over 90,000miles) and still puts in 31.2amps.
Would recommend the B2B, Top Product!!!

In regards to your posting Phillip, I guess that the Hymer equipment would simply overload as the unit's potential of 50Amps would be too great, plus as the B2B is directly connect to both leisure and engine battery (not via existing equipment, or at least it shouldn't be), the digital read-outs on the more modern Hymer's would not meter the input from the Sterling units (volts would be accurate but capacities wouldn't). This is another reason why we fitted a 100amp rated battery computer to monitor everything going on in the van.


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## Boff (May 10, 2005)

Hi Pieter,

there is a large variety of Schaudt Elektroblock models, and in fact most of them are only sold to MH manufacturers, not to end customers. And IIRC there are even some which have a kind of built-in B2B charger. Of course, if you have a B2B charger already built in, then you don't need another one.

Furthermore, most of the problems arising when charging the domestic battery while driving, are not due to the lack of a B2B charger, but to cheap design or cost-saving measures on the part of the MH manufacturer. The alternator is very well able to deliver sufficient voltage and current to charge both vehicle and domestic battery. However, if the cable connecting the two batteries via the split relay is too long and/or too thin, then a too high voltage drop will occur along the cable, leaving only insufficient voltage at the domestic battery. 

Another, possibly more serious, issue is however what happens if the domestic battery is already fully charged, and the engine keeps on running. Then the voltage could become too high. A wet battery would then start gassing (oh no, I've used the word... :wink: ), and during the next routine check water would be refilled, so problem solved. A Gel or AGM battery, however, cannot be refilled, so it could suffer permanent damage from "overcharging". 

Which is one of the reasons why I have decided to stay with wet batteries.

A B2B charger would prevent overcharging. So would the already mentioned cheap design, because the voltage drop prevents it, however also prevents 100% charging in the first place. 

Best Regards,
Gerhard


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## DABurleigh (May 9, 2005)

spudgun - your B2B could only have put up to 45Ah into your battery after an hour's run. Perhaps your 11.7V was immediately after the hairdrier or similar, and the battery terminal voltage then recovers naturally to that reflecting its true residual capacity.

Philip - as to any B2B compatibility issues, this is simply due to how the B2B is wired and whether the battery current monitoring is done AT the battery (as is the case with the better battery monitors) or in a control black box. All that needs to be ensured is that the B2B output is routed through the current monitoring device - hardly a fault of the B2B.

As to the benefit of the B2B you question, I have always said it depends on the user's need or desire to maximise the charge put back into the batteries in a short driving time. Now if the batteries are already low, say at the 50% mark, frankly a simple split charge configuration, with adequately thick wiring to the leisure batteries, is enough to ensure a considerable charge to the batteries. But at the 75% mark the B2B will win hands down over even a good split charge wiring; it is simply Ohm's Law versus the clever B2B approach. And, of course, with wet lead acids you can put more charge in total into them with the B2B (as with an EHU charger) than with an alternator charging regime in most split charge wiring arrangements..

The B2B is a clever piece of kit. If memory serves me right Sterling received a Queens Award to Industry for Innovation on the back of it. That doesn't mean to say, however, that most motorhomers would benefit from it, and even those who do might actually be served better by a third leisure battery. It all depends on the pattern of use of the motorhome in terms of battery capacity, energy budget, and frequency and duration of the driving. All away from hookup, of course.

Dave


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## 117906 (Nov 1, 2008)

I am thinking of getting one, but I have two sealed deep cycle 115ah, not sure now if suitable, can anyone advise please?

Bob


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## JeanLuc (Jan 13, 2007)

*Ref: DAB's last post*

Thanks Dave,

That is the first time I have properly understood the benefits of the B2B. I now realise that I see the 30A + alternator charging current when the batteries are pretty low - say after 3 nights off hook-up in winter. As you say, if I have been off-charge for just one night, the alternator charges at more like 10-15A.

Since we have just changed from 2 x 80Ah Gels to 2 x 110Ah Electrosol I will see how the alternator gets on dealing with the increased capacity. Typically when away for a week or more, we stay in one place for 1-3 days, rarely longer, and often just one night on an aire.

Philip


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## Ifor (Oct 21, 2006)

As has been said the major advantage is with wet batteries where you can charge at up to 14.8V as opposed to the modern alternators 14.4V. 

I have a 50A sterling unit with the remote and also a battery monitor so I can see the actual input and output at the batteries (2*108Ah). The B2b unit is not that efficent (buck-boost never is) and 50A in coresponds to about 32A out. I will get this 32A initaly if I have used more than about 30Ah. A 30Ah discharge will be close to fully charged in 2 hours driving and an hour will get you a lot of the way there. Without the B2b charger and just a good (thick wire) split charge relay setup 30Ah takes more than 3 hours to charge to full and an hour only gets you aboutr half way there. I actualy have the split charge relay still installed but operating off a manusl switch. If I am more than say 60Ah down then I tend to switch off the B2b initaly and manualy switch the relay. I then get 40-45A for a while. Once the rate drops back to 30A I switch back to the B2b as from there on in it will be better.

Ifor


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## snailsontour (May 1, 2005)

> I am thinking of getting one, but I have two sealed deep cycle 115ah, not sure now if suitable, can anyone advise please?


Look on the web site Sterling Power  for specifications or even better phone Sterling. They are very helpful and the odds are you will end up speaking to Charles Sterling himself - he is great as long as you can cope with his language.

We have had our B2B for about three years and think it is a wonderful product for our type of motorhoming - long term touring and wild camping. Indeed, we have been so impressed we have also bought a Sterling Battery Charger and an Inverter. 
Bob


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## pieterv (Feb 3, 2009)

Thanks everyone for the helpful replies.

I now have a better understanding of how things work. It is obvious that a B2B charger with gel batteries is a waste of money as they can only take about 14.4V and the benefit from the chargers comes from the higher voltage they can provide (14.8V).

The difference might not seem much but with the very low internal resistance of batteries it can make a big difference in current. 

Whilst the Sterling B2B can work with gel batteries it would limit the voltage and therefore not provide any benefit. 

The whole idea works because you are using a high voltage that wet batteries can take because you replenish the water lost in gassing, which you can't with sealed batteries.

So, I will pass on them until my gel battery gives up the ghost and then reconsider and maybe go for a non-sealed battery (which would have to be in a different location as my current obattery is inside the living area) and a B2B charger.

Howver, considering that the alternator will charge upto about 75% as quickly as a B2B (or even quicker) it might just be more cost effective to go for a bigger battery (and resign myself to only being able to use the top 25% after mains recharging or a very long drive).


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## CurlyBoy (Jan 13, 2008)

*Battery to battery*

Hi, I have had occassion to speak to Patrick and wasn't too impressed, it was with reference to a solar panel that I had purchased from him, and the output through the regulator. His advice was to remove the regulator and connect the solar panel direct, now the voltage of the solar panel could be as high as 20volts,I measured this voltage myself, and wouldn't think that this voltage would be good for the battery. Am I right?
curlyboy


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## DABurleigh (May 9, 2005)

"I actualy have the split charge relay still installed but operating off a manusl switch. If I am more than say 60Ah down then I tend to switch off the B2b initaly and manualy switch the relay."

Ifor,

I do similar, but I confess originally with a slightly different motive - to maintain a positive leisure battery energy budget when running roof mains aircon on the move, I needed to be able to squirt as many amps as the alternator would give into the batteries, WHEN their voltage is being pulled down by the demand from the aircon.

I kept the split charge relay permanently inline, which also meant that the b2b could only be powered when the engine was running. This prevented any potential b2b running when engine and leisure batteries, in different charge states, were being charged simultaneously by the Victron charger on hookup. Then I added a remotely switched contactor to bypass the b2b.

Dave


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## C7KEN (May 27, 2005)

Hi Dave
How do you power your roof mains aircon on the move? I have one and a 4kw peak invertor. I can run the aircon for 5 mins to get the van temp down but then switch it off and run the fiamma turbo fan which keeps things not so bad temp wise. Although I have ran the aircon for almost 15mins just to see if the batteries could sustain it I have 320amps of batt capacity I felt that the drain was doing the batteries to much harm at 15mins. Also they were of no use untill they were again fully charged and with only a solar panel charging it was too slow so the genny was necessary . Can I assume that you also use an invertor and a B2B charger keeps the batteries fully charged whilst driving then you switch the aircon off on arrival or go on hookup. I keep my fingers crossed that the non true sine wav invertor is not damaging the aircon (dometic) but so far it seems OK. Living here in Spain its nice to cool things down before going to bed otherwise its harder getting to sleep in the hot months


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## DABurleigh (May 9, 2005)

Yes, you have it right. I have a Dometic B1500S, which is mains only, just 220Ah of batteries, and a 1.6kW sine-wave inverter. (The B2B is irrelevant here, other than I need to switch it OUT!).

So for normal driving use, I use the engine-driven dash aircon. 

For normal static use, mains EHU, but only it seems on the rare occasions when we really want aircon AND you aren't in close proximity to other campers who are sitting outside trying to cope in still, oppressive, super quiet conditions.

For static without EHU, then the quiet Honda EU1.0i is too small to fire up the aircon, but IS big enough to keep it running, so the Victron inverter/charger fires up the aircon from batteries, before handing over the load to the genny.

Finally, yes, when driving and wanting to cool down and evaporate clamminess off us to the maximum extent, we have both the engine driven dash aircon and then mains roof aircon powered by the alternator-battery-inverter-aircon chain. At tickover, the system is consuming battery energy to some extent. At normal speeds, without wipers and lights etc (tend to be not simultaneously required!) the alternator just provides enough to the inverter/aircon whilst trickling a few amps back into the battery to make up for the tickover situations. This is abnormal use, and I would not recommend it as a normal use, for the alternator isn't designed for these continuous loads, day in, day out. 

Dave


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## sallytrafic (Jan 17, 2006)

I think reading your earlier post this is the sort of thing you should use Dave,
personal control and no real current limit


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## gnscloz (Oct 4, 2008)

hello thinking of getting a sterling battery to battery charger, does anyone know do i run it alongside the exsisting split charging sytem or discsard the origional split charging system?? thanks in advance


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## DABurleigh (May 9, 2005)

Your choice. See my above post.

Dave


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## vicdicdoc (May 14, 2005)

*Re: Battery to battery*



CurlyBoy said:


> Hi, I have had occassion to speak to Patrick and wasn't too impressed, it was with reference to a solar panel that I had purchased from him, and the output through the regulator. His advice was to remove the regulator and connect the solar panel direct, now the voltage of the solar panel could be as high as 20volts,I measured this voltage myself, and wouldn't think that this voltage would be good for the battery. Am I right?
> curlyboy


- My [very limited knowledge of solar] is that any solar panel over 10w will need a regulator otherwise left connected [in bright sunshine] without a regulator matched to the Amperage of the panel will 'cook' the batteries


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## Jean-Luc (Jul 21, 2005)

I had some issues with the B to B but got little useful help from Charles. One of my concerns was back feeding the 14.8v output of the charger into the vehicle electrics via the original charging circuit. I was unable to remove the split charge relay as suggested as it seemed to be integrated into the original equipment for looking after all things 12v on the habitation side of things. Also, no one could assure me that the MH original equipment would be 100% happy with the 14.8v. 
So, this is what I did.
1. I took a dedicated supply to the B to B directly from the engine starter battery and on to the leisure batteries.
2. I inserted a 'normally closed' relay in the supply cable running from the leisure batteries to the habitation distribution box.
3. I powered the relay from a source which I knew to come live when the engine started charging and serving the habitation area , in my case the 12v supply to the fridge
Result.
My B to B operates totally independent and unconnected to any of the original equipment so I have no worries of any incompatibilities or unforeseen complications.
The B to B works as well as could be expected, for example, 48 hours in freezing La Rochelle last December saw me with 'only' 12.2v having had the heating on continuously plus light and tele/sat box. After a few hours on the road to Mayenne my 2 X 90ah Varta batteries had fully recover and ably supported another 24 hours under the same conditions
It's a gret bit of kit only let down by very poor MH application instructions giving no cognisance to the various equipment/systems fitted by the different manufacturers.
Motorhomes are generally not fitted with bow thrusters


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## DABurleigh (May 9, 2005)

"Motorhomes are generally not fitted with bow thrusters"

:-D

You should be grateful you didn't buy Victron in that case! 

Dave


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## snailsontour (May 1, 2005)

> thinking of getting a sterling battery to battery charger, does anyone know do i run it alongside the exsisting split charging sytem or discsard the origional split charging system??


I run my B2B charger alongside the split charging system. If in doubt talk to Charles Sterling - I think it was either him or the fitting instructions that advised me to just ignore the split charging system.
Bob


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## gnscloz (Oct 4, 2008)

thanks bob, some people seem to put relay in to shut power off to habitation in case it cant cope with the extra input whilst on charge,


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## Jean-Luc (Jul 21, 2005)

I too spoke to Charles Stirling as I was not at all happy about shoving 14.8v at the original equipment in the MH, also the possibility of back feeding the 14.8v to the vehicle electrics via the split charge circuit, I didn't want the engine management to start 'complaining' about what it would see as an abnormally heigh voltage.
Workarounds were suggested that involved 'modifying' the original equipment fitted to the MH.
I decided against those options and opted for the option of isolating the B to B and making the circuit, engine battery > B to B > leisure battery(s) a dead end when the B to B was operational, which was simple and worked perfectly, as is said k.i.s.s.


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