Robin Cassells
New member
Exactly.
A 40 amp max rated alternator, with an 18 amp B to B charger, into a 100 Ah lead acid battery, would seem appropriate. You wouldn't want more than 20 amps, unless the domestic battery was more than 100 Ah.
Battery to battery charging
- Thread starter morgandlm
- Start date
A 40 amp max rated alternator, with an 18 amp B to B charger, into a 100 Ah lead acid battery, would seem appropriate. You wouldn't want more than 20 amps, unless the domestic battery was more than 100 Ah.
Tranona
Well-known member
Why add a charger when the alternator has more than enough capacity to supply anything the battery will accept? It is the acceptance rate that is the limiting factor and this falls off dramatically as the SOC rises. The alternator will already regulate to around 14.5v.
B2B are used in specific circumstances, usually to charge second batteries from one already charged by other means such as an alternator, generator or solar. That is not what the OP needs.
Robin Cassells
New member
Understood 100%.
So why do external regulators such as the Sterling do so much of a better job? Surely that indicates that the alternator alone isn't doing the best job it can.
I am not suggesting that a B to B charger will be a brilliant tool, just that it will be better than no tool at all. It charges with a profile that is designed to maximise charging and battery life.
It would be interesting to see real data on this as we are speculating on theory here.
My concept is that for £135 there is a high risk of longer battery life and better charging. I can't see a downside. Maybe I'm missing something?
Maybe the conversation should be on whether to fit an alternator regulator or not - at a very similar price.
Tranona
Well-known member
They only do a better job with old style alternators such as the one the OP had on his old engine. They output a lower fixed voltage and the Sterling adjusts that so boosting the voltage. They are out of fashion now because the new alternator that the OP has on his new Beta is better regulated. A B2B does not do the same thing as a Sterling. It simple takes the input from a fully charged battery and passes it in a regulated form to another battery. Typical uses are either like mine which is used to charge a bow battery from the start battery or in recent times to provide a regulated and limited charge to a lithium bank from a start battery charged by the alternator. This is to avoid connecting the lithium direct to the alternator which might cause damage because of the high acceptance rate which could exceed the alternator capacity.
The way to get the best out of the batteries in a small boat like the OPs is to have sufficient capacity such that in normal use they are kept above 50% SOC and have an alternative source of charge such as shorepower or solar to keep them topped up when not in use.
Robin Cassells
New member
Aaaah, I didn't realise modern marine internal regulators had improved to that level.
Thanks.
Thanks.
PaulRainbow
Well-known member
I posted this several posts back.
Pye_End
Well-known member
Sterling make B2B charges, and they describe them as multistage chargers - eg increasing the voltage if appropriate. They also describe how the power is taken from the alternator - ie. not affecting the state of the initial battery.
noelex
Well-known member
Modern marine alternators retain a very conservative charging set point. The charging rate can be increased in the latter part of the charging cycle by a proper three-stage charging algorithm with more suitable set points, but the gains on small 40A alternator will only be slight even with a proper external regulator.
Tranona
Well-known member
Yes I know that - I have a 30A one. However the OP only has a 100Ah battery and a 40A alternator so as noelex points out the gain will be marginal. Better to spend the money on a bigger bank and some solar.
PaulRainbow
Well-known member
Despite many posts to the contrary (in this case), you seem fixated on a B2B charger.
Pye_End
Well-known member
It is key in the OP.
I am trying to understand the nature of the detail of the arguments put forward against having one, but detail seems less worthy of exploration than opinion on some of these threads.
PaulRainbow
Well-known member
It's been explained several times. The OP has a modern 40a alternator, fitting the Victron DC-DC charger restricts the current to 18a (not withstanding the charge acceptance rate of his relatively small battery bank), so the benefits of removing the VSR and fitting a DC-DC charger would be minimal to negative.
It's not an opinions, it's how it is.
Robin Cassells
New member
I too am trying to understand the detail.
Members of the panel believe that a constant-ish output of say 14.4 volts will charge just as well as a 3 stage charger that puts out the correct voltage for the state of charge at any given time.
I'm just seeing the B2B charger as a device that turns a constant-ish voltage into a 3 stage charger. Which to my mind will make it charge faster and look after the battery life better. That's why 3 stage charging was developed and has become standard in all good chargers. The implication here is that a modern alternator, as fitted by Beta, will do just as good a job.
Maybe I'm missing out on knowing what this modern alternator does. Could someone enlighten me - Paul maybe? What is special about the regulator? I am under the impression as is @noelex that there is a tendency towards conservative output in order to not over charge during float charge. Which makes perfect sense. Is the output not a compromise between the high voltage desired for bulk and the lower voltage desired for float? I am genuinely interested in how modern alternators do this.
I don't see 18 amps as a restriction or a negative in this scenario - it is a very healthy current - too much more would be unhealthy. The rule of thumb being 20% max.
noelex
Well-known member
Indeed, contemporary regulators still operate with a single voltage set point. Our Yanmar 120 A alternator regulator is calibrated to 14.2V, which is a common setting. This is superior to older alternators that had a single set point as low as 13.8V, but it falls short of the capabilities of a multistage charger, which can be set to higher voltages for bulk charging of lead-acid batteries.
Charging at a higher bulk charging voltage accelerates battery charging, but it necessitates greater complexity. The charging process must be multistage, and the temperature of both the battery and the alternator must be monitored. A reliable external alternator regulator can perform this function, but they come with a higher price tag and require more intricate wiring. The potential gain for a small 40A alternator is modest at best.
As you propose, a battery-to-battery charger can provide the correct multistage charging, but it typically lacks the sophistication of an external regulator. Notably, many of these chargers do not monitor alternator temperature. Consequently, the charging process is not throttled back if the alternator overheats, potentially compromising its longevity. To safeguard the alternator, a smaller battery-to-battery charger is recommended.
Typically, a battery-to-battery charger with a capacity of approximately half the maximum alternator output is employed (20A for a 40A alternator). This reduces the charging current to the maximum charge rate the alternator can deliver when cold. This reduction further diminishes the advantages of multistage charging.
morgandlm
Active member
Thanks to all the people who have taken the trouble to comment about my original question (I am the “OP”). Many of the responses concern the merits or otherwise of B to B chargers. The background is that in anticipation of perhaps installing a LiFePO4 house battery, I had done some research about options for charging it. I found some on-line marketing information and so was thinking that a winter project might be to install a B to B charger and get some benefit while still using a deep cycle lead/acid until it fails and would then be well placed to move to Lithium.
I found the following text on one of the sites selling B to B chargers:-
<< Direct alternator charging used in a traditional split charge system with a VSR will only achieve around an 80% charge state. VSRs simply allow charge to pass directly from the alternator to the auxiliary battery whereas Battery-To-Battery chargers take the alternator output and boost or reduce it to provide a stable voltage output according to a multi-stage charging profile. This provides a 100% re-charge for the leisure battery in a controlled way and typically achieves it faster than with direct alternator charging. Current flow is limited by the charger's current rating, eliminating potentially damaging high in-rush currents and often allowing the use of smaller connecting cable than with VSR-based split charge systems.>>
Now perhaps that text is just marketing hype or perhaps it is someone’s opinion or perhaps “it is how it is”. My own view is that the charging of the house battery would be more efficient and more complete because of the multi-stage profile but accept that any improvement may be marginal and may not justify the expense. But if going to Lithium, I would need to do it anyway. The Sterling BB1225 does monitor battery temperature. Both the Victron and the Sterling are programmable to battery type.
My own experience to date is that charging through the VSR is, of course, significantly poorer than it was when using an external regulator but I don’t want to jeopardise the warranty on the alternator by fitting one. I have never seen any more than 8 amps going in to the house battery even when it is down to 60-70 % capacity so the constraint of an 18 amp B to B charger would not worry me. The 18 amps is less than 50% of the alternator rating which is the recommended limit to avoid overworking the alternator.
Thanks again for all the input - I have learned something but still don’t know which of the two chargers is better!
David Morgan
I found the following text on one of the sites selling B to B chargers:-
<< Direct alternator charging used in a traditional split charge system with a VSR will only achieve around an 80% charge state. VSRs simply allow charge to pass directly from the alternator to the auxiliary battery whereas Battery-To-Battery chargers take the alternator output and boost or reduce it to provide a stable voltage output according to a multi-stage charging profile. This provides a 100% re-charge for the leisure battery in a controlled way and typically achieves it faster than with direct alternator charging. Current flow is limited by the charger's current rating, eliminating potentially damaging high in-rush currents and often allowing the use of smaller connecting cable than with VSR-based split charge systems.>>
Now perhaps that text is just marketing hype or perhaps it is someone’s opinion or perhaps “it is how it is”. My own view is that the charging of the house battery would be more efficient and more complete because of the multi-stage profile but accept that any improvement may be marginal and may not justify the expense. But if going to Lithium, I would need to do it anyway. The Sterling BB1225 does monitor battery temperature. Both the Victron and the Sterling are programmable to battery type.
My own experience to date is that charging through the VSR is, of course, significantly poorer than it was when using an external regulator but I don’t want to jeopardise the warranty on the alternator by fitting one. I have never seen any more than 8 amps going in to the house battery even when it is down to 60-70 % capacity so the constraint of an 18 amp B to B charger would not worry me. The 18 amps is less than 50% of the alternator rating which is the recommended limit to avoid overworking the alternator.
Thanks again for all the input - I have learned something but still don’t know which of the two chargers is better!
David Morgan
Robin Cassells
New member
Yep, all makes sense to me. I understand the benefit is minimal at these currents, but it is a benefit and not a negative. A further benefit - to my mind - is the kindness to the battery. This will reduce the rate of performance loss that all batteries suffer from. At say 2 years down the line, the battery will be in better condition and performance will be better than if an alternator output was used. This will increase the service life of the battery and it's efficiency towards the end of it. I still, maybe naively, believe that the £135 investment gets a reward - albeit small.
Agreed temp monitoring is critical to doing the job properly and feel that the 18 amp charger discussed is a good size if not using temp sensing.
The cost and benefit of something such as a Wakespeed is obviously totally out of proportion with the system under discussion.
Glad some benefit came from the thread. I would suggest that 25 amps is a bit high - assuming you are using 100Ah or less (lead acid) domestic battery.
Pye_End
Well-known member
Of course an undersized B2B could throttle the charging. An excellent reason for getting the correct size, if that is a route the OP wants to go. The OP also knows this - which is why the OP has asked what an appropriate size would be. In itself, the size of the B2B is perhaps not to be a reason to, or not to instal, and unless it can be shown that no B2B can work properly with the limitations of the installation, then it is a flawed argument. Perhaps information on the acceptance of the battery at say 50% SOC would be a good starting point, though looking to the future of lithium perhaps max size to what would be considered a suitable alternator load might be more useful. It may well turn out that 18A (is there a reason you type 'a' not 'A'?) is sufficient for this battery - I don't know, but I bet someone does. Post 4 suggests that 18A will not throttle the system. An important point, but this is the sort of thing where the devil is in the detail.
Modern alternator. Again I ask what the max voltage is of a Beta 40A alternator. Presumably you know, given your resolute conclusions. Relevant, and certainly useful to understanding potential benefits. We all know that charge acceptance improves with voltage, so if there is headroom over what the alternator kicks out then there will be an advantage of some description with a B2B installation. Similarly, if there is no headroom then there will be little to gain.
Multi stage charging. Both you and Tranona seem to ignore the comments made on this. Is there a reason why? Post 35 is typical of other sources of information on the subject and yet doesn't seem to be a factor in your consideration.
PaulRainbow
Well-known member
I think you find find the quote in post #35 originates from Sterling sales literature, hardly non-biased and it's nonsense to say that batteries only get to 80% using a VSR. The quote has been posted on various internet forums, where some people believe everything they read as gospel.
You seem to have all of the answers you need, so i'll leave you to it.
I'll also ignore your facetious comment "(is there a reason you type 'a' not 'A'?)" where it belongs.
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William_H
Well-known member
As paul says 80 % max charge possible with 14.2 volt alternator is completely misleading. It all depends on engine running time. So a sail baot where engine is used minimum time then yes battery will not fully charge. (unless you have a shore power charger as well) But a mobo with longer engine running time probably batteries will always be well charged. As for getting longer life out of a battery kept well charged by a B2B charger well I think that is a wild claim. (perhaps but...) ol'will
Tranona
Well-known member
You need to understand how batteries are used in small sailing boats like the OPs Sadler 26. The typical daily consumption when cruising (assuming no fridge) is +/- 40Ah (roughly double with a fridge). This about 50% of the usable capacity of a 100Ah battery such as the OP has. The challenge is how to replace this. The limitation using the engine alternator is the acceptance rate of the battery which is typically 10A going from 50% SOC up and reduces the closer it gets to full. That is what underpins Sterling's 80% claim. It is not that the alternator cannot charge to full, but as pointed out in post#39 the engine is simply not run for long enough. Earlier the OP said he rarely sees more than 8 amps from his alternator. That is not unusual. I have a 70A alternator and a 190Ah bank and rarely see more than 12A for any period of time. My previous boat had 300Ah and a 115A alternator and rarely saw more than 15A. A B2B will not change this. The charging (bulk) voltage on a 3 stage for an AGM is typically 14.4v - the same as the alternator. The B2B does have a float charge, designed for when the charge source is on permanently. That is largely irrelevant with a sailboat engine as it is only running for short periods when the battery is less than fully charged.
As I suggested earlier to get better use out of the house bank doubling capacity means that not only does the buffer last longer, but the charge it can accept is higher as I describe above. The most important thing is to add generating power when the engine is not being used. the 2 obvious are solar and shorepower. As I said earlier, starting your cruise with a fully charged bank is the key not only to getting the best out of the batteries, but to increase their life because they will spend most of the life close to fully charged and rarely if ever go below 50% SOC. I use shorepower and with my last boat that did a lot of 2-3 days away with no charging other than the engine would take up to 2 days on shorepower to get to float. Batteries were still good after 6 years. If not on shorepower then solar can achieve similar results.
Of course lithium changes all this because it has effectively no acceptance rate constraints when partially discharged and the B2B is one way of controlling the charge to protect the alternator. Its 3rd stage may well come into play because unlike LA it is possible to fully charge with relatively short periods of engine running.
