ACR or DC to DC charger?

[Sailmonkey]

Originally Posted by jms28

Reading this article with great interest and have a question on this particular statement: If ACR connects also when other sources like the shore line battery charger are working, isn't this de facto creating an override and making separate batt outputs for Starter and Service Bank obsolete? As in those separate outputs would normally manage the charging currents for both banks separately (depending on charge, batt type, etc.) but the ACR connects them anyway, potentially leading to the smaller Starter Bank being 'fried' while the Service Bank still needs charging? Or am I totally wrong here? Please educate me

Best,
Mike

A charger with multiple outputs isn’t doing anything different here, it’s just keeping the batteries separated by diodes. Both batteries receive the same charge voltage, just that the starter battery, being fully charged, doesn’t accept any charging current.

[EngineerRetired]

Just sayin...


A healthy starting battery with no load on it should stay charged (enough to crank) for months between starts. (I have diesel tractors sit that long in the winter)

I don't think I would have any sort on interconnect. If you feel you need some sort of back up a cheap 10 amp charger from Wallyworld you plug into your inverter will put enough charge in the starting battery to crank the engine in less than an hour.

Yeah I know, the "oh but in an emergency" thingy. I would doubt you will let the engine sit for months, It's not really an issue.

I am a big fan of KISS

[goboatingnow]

In the case of paralleled FLA batteries , like when a ACR kicks in , then the battery with the lowest SOC will determine the voltage at the terminals of both batteries. Hence current will largely flow into the one deepest discharged , as it’s terminal voltage rises, current will slow into it and some current will then divert into the other more fully charged battery , ultimately both will be fully charged and the current will drop to a leakage value.

Problems can arise where the absorption phase , which has a high constant voltage goes on for too long ( because of one big domestic bank) this can put the smaller starter battery into some distress, including unwanted gas release and loss of electrolyte.

Hence the dc to dc charger concept or completely independent mains chargers approach is better in my experience then paralleling solutions.

( it’s generally not an issue for conventional alternator regulators as they don’t do absorption phases in general and are typically set to fairly conservative voltages )

[Maine Sail]

Originally Posted by Cpt Mark

I too have the Sterling unit with 3 outputs. Each output can be programmed for the chemistry of the particular batteries

No, it cannot be..

Originally Posted by Cpt Mark

and then there is one program that you can define any way you'd like.

Yes, it can be custom programmed, but it is still a single power supply/single voltage regulation circuit that divides the single output three ways. Each of the three batteries gets the same charge profile..

[sailinglegend]

Originally Posted by Maine Sail

er9,

I would urge you to read this article. It helps demystify the numerous myths (many of which have been brought up in this thread) surrounding ACR/VSR/Combiners.....

https://marinehowto.com/automatic-charging-relays/

I agree Ron, everyone on this thread should read your article. It is good to see you posting more here - you're still regarded by all as the most knowledgeable and helpful poster online - but I would like to challenge you on one myth-buster you tried to debunk. - but I think failed.

Myth #3 - overcharging the starter battery - basically all you were arguing was that an ACR does the same as any other charger when more than one output is connected to more than one bank - and chargers aren't blamed for overcharging starter batteries! Well maybe they should be?

You say you haven't seen many 'overcharged' starter batteries - but these are usually open flooded wet LAs that can be easily topped up with distilled water if they have been 'overcharged'.

You also explained how Blue Sea Systems on their more expensive P-Series ACR is switching in an additional diode to the start battery output leg.

Switching in an extra diode causes a 0.6V drop on the start battery output..... a nice selling feature for sure.

This would still quickly charge a starter battery at 13.8v and this 'feature' surely gets around the potential 'overcharging' that many believe is happening because the starter battery would never spend any time at its gassing voltage of 14.4v+.

So let me explore that overcharging scenario using a standard shorepower charger:

Consider this just connected to a 400Ah House bank at 50% SoC. 200Ah needs to be replaced, so depending on the charger current output it may take only an hour to get to the Absorption voltage of 14.4v. It is then going to spend considerably more hours sitting at this gassing voltage until it drops to Float.

Now connect the same charger to a 100Ah starter battery that has started the engine only once so will be at 99% SoC or higher, so less than 1Ah needs to be replaced. The charger voltage will rise very quickly to the Absorption voltage but soon drop down to a safe Float voltage. This may only take in the order of minutes. So to charge a starter battery could take minutes, but to charge a house battery from the same charger could take hours.

So combine the House and Starter banks - any way you want - and the Starter bank is then sitting for hours at a gassing voltage of 14.4v+ - not for minutes. It will not be overcharged by taking too much current - it will be down to zero very quickly - but it is being overcharged by sitting at a gassing voltage for too long and losing water rapidly.

So overcharging a battery is surely defined as 'applying too high a voltage for the SoC of the battery."

Other reference sources which confirm this definition:

Nigel Calder - Boatowner's Mechanical and Electrical Manual:
If the absorption phase were to be continued beyond the fully charged state, the battery would be damaged from overcharging. To avoid this, at the completion of the absorption phase, a multistep regulator trips to a lower constant-voltage float setting (generally between 13.2 and 13.6 volts). This protects the battery from overcharging during periods of extended engine-running time.

And all battery manufacturers say their batteries must be dropped down to Float voltage when fully charged to protect the battery from overcharging.

Ron, please correct any misunderstandings I have made here.

By the way my starter battery is charged by a 10 watt solar panel, via a Morningstar Solar regulator, and was recently replaced after 14 years on a Liveaboard boat in the Med - just because the AGM house bank was retired too.

[JArcherM]

Originally Posted by er9

thanks for posting the diagram. food for thought.



my mind wonders though...what scenarios/ events could cause that 300AMP fuse between the common post and the distribution panel to blow? if it did you would instantly loose power to anything connected downstream from the panel.



i guess that wouldnt be the end of the world because necessary items like VHF and bilges would bypass the distribution panel.


what is the likliehood of that 300 AMP fuse blowing? has it ever happened to you?

Fuse is there to protect the primary wiring from dead short. It needs to be there per abyc standards and needs to be able to disrupt the total available current of the battery. The leads coming off that bank are quite large and the fusing needs to be rated for that wiring regardless of what is downstream. Anytime the wire size changes smaller, the smaller wire gets its own fusing.