Charging batteries in 'absorption' phase

[M&H]

Hello all --

 

I'm just about to replace my battery bank, but I also have a question about my charging system.

 

We're planning to get either 270ah or 405ah capacity. Given our current power consumption (about 40ah/day), a new bank should mean that we'd always be above 80% SoC. If I understand things right, then that'll mean we won't need to bulk charge them -- they'll always be in absorption or float stages.

 

We've got 200W solar panels, which work brilliantly right now. But we've also got an alternator on our BMC 1.5, which several engineers have seen and said it's a 'standard' 70a alternator. I've tested it regularly and never measured an output above 19a though!

 

If we don't expect to bulk charge our batteries, would this still work effectively as the output would be lower? Could the low alternator output be related to the near-death condition of our current batteries?

 

And about the battery bank - many have recommended getting a larger bank, but given our usage, is it worth it? Would it be easier to replace, say, 60a in a smaller bank, as some of the charging would be below 80% SoC, and so the batteries would accept a higher charge at the start? Also, wouldn't it be easier to get the bank up to 100% with a smaller bank?

 

Many thanks!

[bottle]

I'll start, whenever you charge batteries they start to charge in 'bulk' then 'absorption' then 'float'

Some links that may help you understand the ways of batteries and charging

 

http://www.batteryfaq.org/

 

http://www.smartgauge.co.uk/technical1.html

 

and particularly this page on charger sizing

 

http://www.smartgauge.co.uk/chargesize.html

 

No doubt others will add to this

[Ex- Member]

Your low alternator output is probably due to poor pully ratio. I suspect you'r just at a 2 to1 ratio hence the low amps.

 

I would surprised If you could sustain an 80% SOC especially through winter unless you're strictly 12v with no fridge.

 

From what research we've done and what we've monitored over the past year is that a battery bank needs to be about 4 x your daily consumption. That is working extremely well for us, but of course might not for everyone. IMO though it's a good guide line.

[Robbo]

 

And about the battery bank - many have recommended getting a larger bank, but given our usage, is it worth it? Would it be easier to replace, say, 60a in a smaller bank, as some of the charging would be below 80% SoC, and so the batteries would accept a higher charge at the start? Also, wouldn't it be easier to get the bank up to 100% with a smaller bank?

 

 

I would recommend a higher battery bank if you use the boat in winter, you can then go for longer periods of not charging. If you only use in summer go for the the lower battery size capacity and use leisure batteries.

[smileypete]

If you take say 50Ah out of a fully charged 100Ah bank, and the same from a fully charged 400Ah bank, then recharging the latter can be a be a lot quicker.

 

But to be a lot quicker you do need a higher power charge source. And of course a lot more batteries! Which all costs money.

 

If you use a smaller charge source for both the time difference won't be so marked. For batts in daily off grid use a good strategy can be to do a majority charge each day for 1-2hrs, then a much fuller 6-8hr charge at least weekly.

 

Much depends on budget and circumstances...

 

cheers, Pete.

~smpt~

Edited July 22, 2014 by smileypete

[nicknorman]

The distinction between bulk and absorption is often misunderstood. It is less to do with the batteries and their state of charge, and more to do with the charging device (though, as I shall explain in a moment, it's a bit of both)

 

Bulk charging phase is constant (maximum) charging current, limited by the charging device and presumes that the batteries are able to take that charge. But of course, if the battery bank is large and the charging device small, bulk charge phase will continue until the battery is approaching fully charged. Only when the battery charging voltage approaches the regulated voltage -because the battery stops being able to take the max current - does the bulk phase end.

 

The idea that bulk charge occurs below 80% SoC is a flawed one due to this reason (ie it depends on the relationship between battery size and charger size, but also as follows:

 

If you take an average sort of battery bank and an average sort of alternator, and start charging from 50% SoC, then yes typically bulk phase ends at around 80%. However, if you take the same setup with fully charged batteries that are then discharged to 80%, then start charging, the charging will be in bulk phase for a good while. This is due to the surface effect - you have depleted the chemicals on surface of the battery plate during the discharge, and that charge / chemicals are easy to replace being on the surface of the plate. In other words, the battery is able to take much more current at 80% SoC if you arrive there from a discharge, than if you arrive there from a prolonged charge (whence the surface chemicals are all fully restored and the charging reaction has to take place deeper inside the plates).

 

All that is another way of saying that you will get "bulk phase" charging from a bank discharged to 80% SoC. BUT it seems your problem is inadequate alternator output - definitely something wrong there which should be addressed before you knacker your new batteries. You may be able to rely on solar in mid-summer but surely not over the winter half of the year.

[by'eck]

By design multi-stage chargers will always start in the bulk (constant current mode) until the pre-set absorb voltage is reached.

 

I even get this on fully charged batteries previously left on float. If I cycle charger off/on the bulk phase lasts for 20 seconds and battery volts momentarily hit 15.2 volts before dropping back to the pre-set 14.8 volts absorb voltage, since the switch conditions from one phase to another is usually fairly soft.

Edited July 22, 2014 by by'eck