B2B, have I understood?

[costaexpress]

Long post and I would be very grateful for any pointers. I have two 105AH batteries (Lead acid), a token 40w solar panel and a 50l compressor fridge. I can last 2 nights, 36 hours, no problem off grid. I use my van for hobbies not touring or holidays and so I never stay longer than 2 nights anywhere. I travel all over Europe, however, usually only travel around 60 to 100 miles each day once I have reached my destination. Whilst this puts a little juice back into the batteries by the time the Ducato alternator has replenished the cab battery and kept the compressor fridge going there is little juice left for the leisure batteries on a say 60 mile drive given that I could also be parked up for many hours with the fridge still going - is that assumption correct?- I currently resolve the issue by mixing ehu stops with remote stopovers, usually car parks of archaeological sites. I believe installing a B2B system would pump up to 50AH's into the batteries for every hour driving - is this a correct assumption? If I also invest in two Lithium batteries at the same time I believe my PX300 mains charger will cope and that the Ducato alternator wont know any different whilst driving - are these assumptions also correct? If anyone is still awake by this point I would appreciate any pointers before ordering and arranging installation (probably rhino installs). Solar is not my preferred option as my travels are heavily skewed to the winter months, the summer spent on holidays and breaks with wife and family, and a long shot solution may be to replace the compressor fridge with a 3 way, however, cant find anyone who does this installation at the moment.

[silverback]

hi, I am thinking of going to Lithium the px300 from sargent specs say that it can cope with the charging of the lithium, I contacted sargent for information but as yet they haven't got back to me, these batteries get a decent review https://www.ksenergy.co.uk/store

not much help i know but thats all i have got at this moment in time! :-S

Jonathan

[spirou]

Think of a B2B as a mains charger, except its source isn't a socket somewhere but the alternator. But the battery won't see that difference. However, a B2B won't reacharge your hab battery much (if any) faster than without one. Bigger cables would be a better investment. Unless you have a newer vehicle with a smart alternator in which case a B2B is pretty much mandatory.

 

Also mandatory if you hab battery chemistry is different than starter battery. So if your hab battery is Gel/AGM and especially LiFePO then you need one as they require a different charge profile than a regular flooded starter battery.

 

Charging rate will also depend on B2B model (some provide 25A, others more or less) and the battery itself. With rising internal resistance as it gets charged, the current will fall and it will be rare that you see full output for very long. I have 180Ah & 25A charger. After a typical nights use I might see over 20A going in for less than 15min then a gradual decline. A lithium battery will sustain higher currents for much longer thus charge faster. But there's a whole bunch of potential issues if you choose that path. No matter what anyone tells you, it is very rarely a straight drop in replacement.

 

 

 

[arthur49]

spirou - 2020-11-21 2:34 PM

 

...... No matter what anyone tells you, it is very rarely a straight drop in replacement.

Yes.

 

Lithium is great but needs to be carefully planned otherwise there is the potential to knacker your alternator, mains charger, cabling etc. One big advantage of lithium is the ability to fast charge. One big disadvantage of lithium is the ability to fast charge.

 

Check this out

 

[costaexpress]

spirou - 2020-11-21 2:34 PM

 

However, a B2B won't reacharge you hab battery much (if any) faster than without one. Bigger cables would be a better investmentt.

Thanks spirou, i had thought i could use a B2B to charge hab batteries quicker when driving as I thought the vehicle alternator cut back once the cab battery was charged and the B2B overcame that by fooling it into thinking it was one big battery. Glad I asked the question now before I wasted my money!

[costaexpress]

silverback - 2020-11-21 2:21 PM

 

hi, I am thinking of going to Lithium the px300 from sargent specs say that it can cope with the charging of the lithium, I contacted sargent for information but as yet they haven't got back to me, these batteries get a decent review https://www.ksenergy.co.uk/store

not much help i know but thats all i have got at this moment in time! :-S

Jonathan

Thanks for the link, great price, will let you know if I go that route

[steve hill]

Wow and whoa. Thats a lot of info, but, I have used solar and B2B for 20 years or more, so should just say a few words.

1. Lithium batteries are very expensive and like all batteries will fail in time.

2. 3 way fridges last for 20 years use gas and are expensive but are a one off payment.

3. B2B WILL charge your spare battery once the engine battery is charged which should be quickly if you are using it at least once a week and at a very high rate depending how much you pay and on your alternater output.

4. 40 watt panels are not worth it, get at least 120watt and it again will last for at least 10 years.

 

So if you can afford it skip the lithium, change the fridge, up rate the solar and get a good B2B fitted and your problems will be solved for the next 10 years minimum....this advice is from 25 years of fulltiming experience on 2 vans.

 

[kevina]

B2B Will recharge your batteries quickly and sounds ideal for your requirement, you just need the right one with proper installation. I don't see lithium batteries giving you any particular advantage, max output of 45/50A is what you will get with any half empty 200Ah battery bank for at least an hour.

[costaexpress]

Thanks everyone, however, just a little confused now, there seems to be a difference in opinion as to whether B2B will make any difference and that I maybe should get some bigger cables, for where please, to connect what to what?

 

On the other matters raised, thanks for the comments about lithium batteries, I will investigate the 'plug n play' aspects and what else I may need to do. The 40w solar was on the van when purchased and its main use appears to be to keep the cab battery fully charged in storage which it does well, however, of no use for the compressor fridge in winter. Not following solar route just yet as my travel is skewed towards winter months. My last van had a three way fridge which was very flexible, however, to retrofit to new van involves cutting into van for venting, adding gas supply etc and I would need to get it done professionally, in addition the compressor fridge makes the 3 way look like a toy in terms of performance, against of course is all the power management issues I have to think about.

 

Anyway for now need to sort out just what a B2B does as I don't want to put even more equipment in if it doesn't help. (Balance of replies certainly suggests it will)

[Derek Uzzell]

There is a fair amount of B2B past discussion on this forum, plus this advice on the late-Allan Evans's website

 

http://www.aandncaravanservices.co.uk/battery-to-battery-chargers.php

[mikefitz]

A battery to battery charger, correctly installed with suitable cables, will charge your battery much quicker than the existing split charge system. With the existing system a charge voltage of typically 14.4 volts will never be reached at the battery, during the high current phase of charging, due to volt drops in the citcuit and the less than ideal voltage output from the alternator. Even a small volt drop of 0.2 volts could halve the charge current. Regardless of the input voltage to the B to B, the output will be controlled at the correct charge voltage. The B to B will get the batteries to around 85% of full charge quite quickly. However it will take a long time, around 5 hours, to 100% charge a half discharged battery bank, even with a powerful charger.

As the batteries approach full charge the internal resistance to charge current increases, reducing the current accepted by the battery to a lower and lower level as full charge is reached. This is a function of the lead acid batteries not the charger.

The recommended charge current for a lead acid battery is 10% to 30% of capacity depending on battery type, GEL batteries at the lower current, AGM at the higher.

With a 30 amp B to B charger set with the correct charging voltage, a typical 200 Ah battery bank discharged to half capacity ( 100Ah) would charge at 30 amps for the first two hours getting to around 80% of capacity. The batteries reluctance to accept current would become greater during the third hour of charging, perhaps only allowing 15 Ah into the batteries. To reach full battery capacity it will need a further 2 hours or longer for the last 10%.

In practice its acceptable to cycle the battery between 90% and 40% of capacity provided a full charge is provided by mains or solar charger every few weeks.

 

40 watts of solar is not enough to to support a compressor fridge, ideally at least 200 watts of solar with a quality MPPT solar controller is needed.

 

Changing the compressor fridge for a 3 way may be impractical, given the layout of the van. The cost of conversion could be in excess of upgrading the electrical system.

 

Lithium batteries would offer ideal charging, unlike lead acid they charge at a constant rate and dont need to be fully charged. In fact the life is prolonged by operating within the capacity range.

 

The installation would require modification to the Sargent system, a battery to battery charger and a suitable mains charger. ( the existing Sargent charger is not really suitable). Cables and fuses would need updating. Although 'drop in' lithium batteries have internal battery management systems, BMS, this should be regarded as a 'last resort' protection for over charging or under discharging. Every time you 'hit' the BMS limits the battery is degraded slightly.

The most important limit is over discharge, adding an independent protection device is recommended , such as the Victron battery protect,

 

https://www.victronenergy.com/blog/2016/03/08/batteryprotect-it-does-exactly-what-is-says-and-more/

 

The Lithium batteries I would recommend currently available in the UK are,

Sterling ,

https://sterling-power.com/products/lithium-batteries?variant=50393785109

Victron,

https://www.onboardenergydirect.co.uk/shop/Victron-Lithium-SuperPack12.8V-100Ah.html#SID=108

Relion

https://www.alpha-batteries.co.uk/12v-100ah-relion-lithium-battery/

 

Recommend battery to battery chargers with outputs in the range 30 to 60 amps are available from Sterling, Victron and Votronics.

 

For the installation I would recommend,

https://www.vanbitz.com/

 

A point to note with Lithium batteries is that charging is not possible at low temperatures, it causes irreversible damage to the batteries. ( you can still draw power down to -20C, but cannot charge below 0C) Although some Lithium batteries have a disable built into the BMS, many dont and its important the chargers have a low temperature disable built into the lithium profile.

There are workarounds to this involving heater pads if you intent using the van at low temperatures.

 

Your current battery bank is 200 Ah, its possible that a single 100Ah or120Ah lithium battery would be enough for your style of traveling.

 

A useful additional item to any battery system is a battery 'fuel gauge', a battery monitor. This will indicate the status of the battery bank, current and voltage, amp hours stored and more. The most accurate and comprehensive are Vicrton BMV series and the Victron Smart Shunt.

 

Mike

 

 

 

[costaexpress]

 

Wow! Thank you, brilliant piece really helpful and has convinced me I will take the van to eg rhino installs, vanbitz or similar to make sure everything matches rather than trying to do it myself

[spirou]

Pay special attention to that last part about low temperature charging and how that might impact your use since you say you mostly travel during winter. Also, I believe Victron and a few others cut off at 5°C rather than 0.

[colin]

I would note that there are now lithium batteries which will charge down to -20c, they use their own internal heaters, but they need to heat themselves up before charging starts, this makes them more suitable for the user who wants to extend into winter rather than mainly use in winter.

[silverback]

I've got to say, buy a lithium with in built battery master system, this prevents overcharging and other things, and they don't fry alternators that is Bull

Jonathan

[insider]

I've a 2016 Forte(184) with the PX300. I've changed the batteries to 2x90AHr LeadAcid and have 90Watts solar. Based on this thread https://www.swift-talk.co.uk/swift/forum/motorhome-general/310728/b2b-charger-on-a-forte I'm after purchasing a reconditioned 60A Sterling B2B and will be fitting it in the next few weeks. I have a three way fridge but in other respects my setup is similar although my usecase will likely be different. From memory I think there is a 25Amp fuse in the EC400 so that is the max it can handle taking power from the vehicle battery. Given that while driving your fridge will be using a few amp then your batteries won't be getting much. As in the thread I am fitting a relay to disable power from the split charge relay whilst driving so the voltage from the vehicle battery doesn't confuse the SterlingB2B and I may even fit a relay to the solar as well. I have all the bits and need to add a venting tube to the Varta LDF90s as even though I am doubtful they will need it better to be safe than sorry. I'm staying away from Lithium for all the reasons mentioned in this thread and am a Euro 5 engine. If you're Euro6 then you really do need a B2B. PM me and I'll let you know how I get on in the new year

[arthur49]

Insider

 

On the basis that you don't have a smart alternator I would be inclined to simply beef up the wiring from vehicle to leisure batteries, rather than spend money on a B2B.

 

I did this on a previous van and it made a huge difference to charging whilst driving. From memory the voltage at leisure batteries rose from 13.8v to 14.34v. I don't think there is any need to tamper with existing split charge relay and wiring - leave it in situ. Basically add a second cable with its own beefy relay, bypassing the EC400, and you should see a big improvement (I used 25mm2 cable but, depending on the length of run, you'd probably get a big improvement with 10mm2 or 16mm2)

 

Check out this from the late Allan Evans' website: http://www.aandncaravanservices.co.uk/add-a-second-battery.php

[costaexpress]

Thanks everyone, I guess I was a little naïve hoping for a straightforward answer, seems to be yet another one of those dark arts associated with motorhoming, looks like the science can swing in either direction so I have decided to take a punt and fit a B2B anyway, apologies to all those who advised against.

[spirou]

It's not either/or. More like, does it make sense for the price and expected use? That depends on each case. Change one element and you can have a completely different requirement.

[insider]

arthur49 - 2020-11-23 6:20 PM

 

Insider

 

On the basis that you don't have a smart alternator I would be inclined to simply beef up the wiring from vehicle to leisure batteries, rather than spend money on a B2B.

 

I did this on a previous van and it made a huge difference to charging whilst driving. From memory the voltage at leisure batteries rose from 13.8v to 14.34v. I don't think there is any need to tamper with existing split charge relay and wiring - leave it in situ. Basically add a second cable with its own beefy relay, bypassing the EC400, and you should see a big improvement (I used 25mm2 cable but, depending on the length of run, you'd probably get a big improvement with 10mm2 or 16mm2)

 

Check out this from the late Allan Evans' website: http://www.aandncaravanservices.co.uk/add-a-second-battery.php

Good advice. However already bought the B2B at a good price and it will only need 2.5m of wiring which will be 16mm2. If I had to run new cabling from vehicle battery to EC400 and back to hab battery it would be at least 8m and very very difficult. Also and this is important for the OP, the split charge relay is going to treat both batteries the same. The B2B will have smart charging and give a better charge to the hab batteries.