How much converter/charger is really needed?

[Bobbo]

I have a pair of NAPA 9824M Marine/RV batteries with a 20 hr rating of 79AH each, wired in parallel.

Is there, really, any advantage in going from a 45 amp (PD4645V) converter/charger to a 55 amp (PD4655V) converter/charger?

I know that bigger is ALWAYS better, and the cost is not much more, but, really, is there a real advantage?

If not, how many AH do you need to make the jump from 45 amps to 55 amps useful?

[DrewE]

BFL13 wrote:

naturist wrote:
Bigger is not always better. It is possible to have too much charger, drastically shortening battery life. Two batteries each capable of absorbing 35 amps would be fine on a 55 amp charger. One such battery, however, would be in trouble.

My understanding is that battery will only absorb what it can and it will not be damaged. The converter will only supply what amps the battery will take. No harm done.

The waste is if the converter you bought for a bunch could supply 100 amps and the battery will only accept 40 amps when a 40 amper costs a bunch less than the 100 amper..

Certainly with any halfway modern converter (and the PD units all certainly qualify), the converter basically just acts as a regulated voltage source with current limiting. A 70A converter and a 45A converter both putting out 14.4V will charge a little battery at exactly the same rate, since the charge voltage is the same; the 70A one will just be underloaded and loafing along more than the 45A one.

In short, the battery will not be damaged. You could hook the biggest converter up to a motorcycle battery and it would be okay. It might stay in bulk charge mode a little longer than necessary, given how the PD algorithms seem to work, but there's no great harm there.

[BFL13]

naturist wrote:
Bigger is not always better. It is possible to have too much charger, drastically shortening battery life. Two batteries each capable of absorbing 35 amps would be fine on a 55 amp charger. One such battery, however, would be in trouble.

My understanding is that battery will only absorb what it can and it will not be damaged. The converter will only supply what amps the battery will take. No harm done.

The waste is if the converter you bought for a bunch could supply 100 amps and the battery will only accept 40 amps when a 40 amper costs a bunch less than the 100 amper..

[naturist]

Bigger is not always better. It is possible to have too much charger, drastically shortening battery life. Two batteries each capable of absorbing 35 amps would be fine on a 55 amp charger. One such battery, however, would be in trouble.

[BFL13]

From the OP's recent posts, it looks like he has a 2000w gen so can go to max of 75 amp converter/charger.

He also wants more solar as though going off grid more, which also means more gen recharging when not enough sunshine. He is also getting new batteries, since he said he can't get through one night on his exiting two older batts with the furnace up high enough to not freeze to death.

He is also a big PD fan, despite the existence of PowerMax! (Weird 🙂 )

So, since the PD 70 amper can be run on a 2000w gen (just) and that is the right charging amps for about 225AH of bank, which he needs to run that furnace, and he will be off grid more---then it is clear:

-Get two Trojan 225 AH 6v T-105s ( or equivalent at Sam' or Costco)
-Get a PD 70 amper to replace the 45
-Get the extra solar
-Get a 300w or so inverter if not already in hand, so now you can run it off grid with the bigger battery bank and watch more TV/DVDs or whatever.

Too easy! 🙂

[theoldwizard1]

agesilaus wrote:
Bigger is not always better, bigger could be less efficient and waste more power. Converters/Inverters are not very efficient in the first place ...

That is not true in today's good quality converters/inverters. Their efficiency is commonly 90% or better, depending on operating conditions.

[BFL13]

If you have a 1000w generator that runs the 45 amper, that gen will not be able to run the 55 amper. If you have a 2000w gen you are ok going to the 55.

On 158 AH starting at 50% SOC, there will be only a few minutes difference in charging time to 80%.

If you decide to get a higher AH battery bank such as a pair of 6s, now the 55 amper would be better (although a 75 amper would be just right.)

If you don't do much generator charging but are on shore power or solar mostly, then IMO not worth changing to a 55.

[time2roll]

Do you expect a fast charge from a generator? For the few $$ more get the 55.
Even for a fast charge into a wet cell battery 20 amps per battery is fine.


Mostly plugged in? 35 amp is more than enough.

Usually the house load is under 5 amps so forget that unless you have multiple tank heaters etc camped well below freezing.

[agesilaus]

Bigger is not always better, bigger could be less efficient and waste more power. Converters/Inverters are not very efficient in the first place that's why they have those big fins to bleed off the waste heat.

[Lwiddis]

Once the battery charging enters the absorption phase they won't accept big numbers. Charge with solar and you can "forget" all this anyway.

[2oldman]

You could charge, theoretically, those 2 drained to 50% in under 2 hours...bulk. I see no advantage.