Battery charger...the "correct" size

[malyea]

35 - 40 - 50 Amp charger...it doesn't really matter, does it?

I have two 12V 8D, wet cell, 200 Ahr batts in parallel providing 400 Ahrs as my house bank (I typically discharge no lower than 50% and currently boat is frequently connected to shore power most of the time).

I need to replace the batt charger and a fair amount of research indicates its not really important that I "correctly size" the new charger to my batt bank...

- because any charger between say 35 to 50 Amp capacity will work fine recharging my two 8Ds - the only real difference is that the 35 Amp charger will possibly take just a bit longer than the 50 Amp charger to recharge the batt bank back to 100%....correct?

[a64pilot]

Yes, to a point. First don’t forget to subtract your house load from the charger size, so if your using 10amps, the first ten amps from the charger is just to break even, then your 35 is only a 25, but the 50 is a 40
Then some battery chemistries do better if they are hit pretty hard during recharge, like AGM and even regular ole flooded batteries may benefit as being charged hard can get the electrolyte stirring around preventing stratification.
Then if your ever away from the dock and on generator, a larger charger can reduce gen run times somewhat, however since battery acceptance rates fall off so fast, it may not reduce total charge times by as much as you may think.

Sorry to talk around in a circle, I guess biggest thing is to not to forget to subtract your house loads

[markwesti]

10% of AH is what I understand , house load is a excellent point . We are thinking of one of these .https://www.ebay.com/itm/Sterling-Pr...19.m1438.l2649
Also here is a good read .
Installing A Marine Battery Charger Photo Gallery by Compass Marine How To at pbase.com

[john61ct]

No 20-30% of AH even with flooded, take into account may upgrade the bank later on.

AGM 50-60%.

And not necessarily much faster overall, but when powered by the genny (still need some solar) any reduction of daily runtime is worth the extra investment.

Plus required for AGM longevity.

[Locquatious]

Make sure that your battery charger has voltage settings (bulk charge voltage and float voltage) that match your battery type or battery vendor recommendations at the various average temperatures.

Monitoring amps may help for reasons given by others, but maintaining proper voltages will assure longer battery life.

[john61ct]

Best is custom adjustable profiles, future proof for whatever chemistry bank down the road.

Sterling ProCharge Ultra or ProMariner Pronautic P

[sailorboy1]

Unless you plan to regularly discharge to less than 60-70% it will make little difference whether you have a 60 amp charger or a 40 charger. Even if using your generator the real world run time difference will only be 20-30 minutes. If you are spending a day in a marina on shore power it wouldn't any difference at all as the charger has all day.

This is personal real life experience.

[john61ct]

Yes obviously if always on shore power 10-15% is fine for flooded.

But 20-40% still needed for longevity for AGM.

[newhaul]

Dyno batteries recommends charge rate of 6% of 20 hour ah rating up to 10% is allowed .
https://www.google.com/url?sa=t&sour...D6QV-maNrqIf_V
( deep cycle batteries)

[newhaul]

East Penn is a bit ambiguous in their charging specs.
Here is what they say .
Proper charging is essential for maximum battery life. In gener-
al, the proper charging rate for lead-acid batteries is any rate
which doesn’t produce temperature higher than 115°F (46°C),
and any rate which doesn’t cause excessive gassing.
https://www.google.com/url?sa=t&sour...tZN-24JajLBc4J

[denverd0n]

To kind of summarize some of the things already said...

10% of total battery capacity is considered a minimum for effective charging of lead acid batteries. A bigger charger doesn't hurt, but getting a monster charger for a small battery bank is rather a waste of money. Of course, if you intend to increase the size of the bank in the future, then a bigger charger might be a good idea.

You pays your money and takes your choice.

[conachair]

Originally Posted by denverd0n

10% of total battery capacity is considered a minimum for effective charging of lead acid batteries. .

Anyone know where or why this bit of perceived wisdom came about?
Can see it for trojans where the majority of t105's have a hard life and any less could lead to batteries not getting to charged over night. But when there's lots of time do the batteries know or care if the current going in is less?

[john61ct]

Yes certainly for AGM as stated above.

Have not seen minimums for FLA nor statements that longevity is affected, but you do want to get to true 100% Full.

Equalization protocol, which I only do manually I believe benefits from higher rather than very low current rate.

[john61ct]

Note unless it were a critical longevity factor, mfg would not state minimum charge rates, since that implies their product is not suitable for millions of potential customers.

[newhaul]

Originally Posted by denverd0n

To kind of summarize some of the things already said...

10% of total battery capacity is considered a minimum for effective charging of lead acid batteries. A bigger charger doesn't hurt, but getting a monster charger for a small battery bank is rather a waste of money. Of course, if you intend to increase the size of the bank in the future, then a bigger charger might be a good idea.

You pays your money and takes your choice.

I have to question the 10% to be the minimum for charging .
I have just provided the spec sheet from a major deep cycle battery manufacturer who's stated that optimum for their batteries was 6% of ah capacity. I actually have never seen the 10% number actually stated in any manufacturer spec sheet for Fla charging.