Battery Chargers and Bulk Charge Rate

Your car alternator puts out more than 30 Amps most of the time way over 100 Amps so with that capacity you can charge your battery and run lots of gear off the battery at the same time.

But I have always been of the opinion a trickle charge over night is better than a quick high Amp charge for a few minutes.

Cheers

Ken
The recognised maximum charge rate is 1/10th of the capacity, IE a 100 amp hour should be charged at 10 amps. The standard voltage of 14.2 is chosen to deliver that 1/10th rate. Eric

G'day Ken
The AGM batteries don't like to have their charge rate more than about 20% of the amphour rate. Probably for good life to run them at the full 20% eg 20 amps for a 100AH battery is pushing it and less rate will be better but it will take longer to charge.

If high amps are pushed into the battery from an alternator or other source then the battery will overheat internally and damage the plates.

Overall, the bulk charge rate must not be too high and the charger shouldn't overheat the battery before it gets to the threshold voltage where diminished or trickle charge begins.

Ross M

Lest start with illiminating AGM as being an issue in the discussion by saying there is very little difference simply because of the battery being AGM.

However some batteries and in particular some deep cycle AGM batteries have quite low maximum charge rates for the battery size.

now that is out of the way.

There is and has been some discussion about optimum and maximum charge rates, some fundamental misunderstanding and a whole pile of complete and utter rot that circulates

Almost with out exception and across almost all battery types the optimum charge rate is considered to be arround 1/10 of the amp hour capacity in amps.

This charge rate gives the most expedient charge rate while incurring the minimum charging losses, optimim minimal heating and minimum gassing and loss of electrolite.

There have been various people that have pushed the idea of trickle charging of batteries at very low current ( note trickle charging as opposed to float charging) sorry my view is that their argument holds very little water.

It is unavoidable that there are charging losses......we need to apply sufficient charge current such that the charging losses do not represent a large portion of the current supplied...back to the 1/10 AH capacity.

With lead acid batteries the voltage determines the rate of charge and the vigour of the chemical process...the current is a result ( stay with me)

If we apply a fixed charging voltage ( traditional charging) the amount of current that flows and the charging effect will be highest at the beginning and will reduce as the battery reaches fully charged....

NOW this reduction can be quite considerable.
If we have a charger capable of delivering a large amount of current in relation to the battery.
The initial charge current may be quite high but after a short time ( shorter than some wish to believe) the current will drop to a level well below that the charger is capable of delivering, and it will continue to drop.

This is why we now have multistage chargers.....they hit the battery harder in the initial stage to speed the charge rate, as the level of charge approaches full charged the charger will drop back the charge rate, if it did not the battery would gas excessivly, resulting in loss of electrolite and excessive heating.

now back to the limits of maximum charge rate.
This varies from battery to battery.
Unsealed wet cell battereies will tolerate very high charge rates compared to their capacity.....as long as you keep filling them with water.
Perhaps in excess of 1/2 the AH rate in amps.
Sealed batteries ( including AGM) generally will tolerate less.....if they are charged excessivly hard they gass excessivly the vents that all of them have, open and gas and electrolite is lost....in most this can not be replaced...continue and the battery fails.

How much sealed batteries will tolerate varies
Ya typical small selaed lead acid battery will generally be specified at 1/3 AH capacity in amps as maximum charge rate

You can not rely on generalisations so particularly with AGM you should check the specified maximum initial charge rate.

NOW it is claimed and reasonably, if you charge are an expedient rate it can reduce sulphation.
It is also claimed that if you charge excessivly fast, excessivly long, or in hot condition this can accelerate sulphation.
Sulphation is one of the major causes of battery failure, in fact if no other cause claims the battery first, sulphation is a sure as death and taxes.

OK one final sacred cow.
If you have a 100 amp car alternator hooked up to a reasonable sized car battery it is unreasonable to ever see 100 amps flow into the battery...even with the battery completly flat and the engine revs high, it is unlikly to ever happen......other factors will limit flow.....after a short time the charging current is likly to be more like 40-30-20 ish amps.

SO what you realy want to know is how big a charger should I buy?

In all practicality just a little larger than 1/10 of the AH capacty of your battery in amps is reaonable, going much over 1/5 of the AH capacity in amps will achieve little.

AND less than the specified maximum initial charge rate for the battery.

I hope this was worth the read.

cheers

Hi Ken,

there are two detrimental effects of high charging currents which are interrelated.

The primary effect is heating caused by a higher internal resistance to the charging current (versus lower resistance to the discharging current).
The amount of heating rises with the square of the current (Joule heating), and as the SOC rises, so does the resistance, and then you have chemical heating (heat of dilution), so there are a number of factors all contributing to an internal temperature rise.
High temperature is bad in a number of ways: formation of corrosion products on the positive electrodes (more oxygen is being tied up in a number of side reactions, thus it can't drift to the negative electrodes where it is needed to oxidise the hydrogen, which is then vented).

High temps subsequently lower the voltage threshold for electrolysis, meaning the battery starts to gas earlier in the charging process.

So there you have it: if the charging current isn't limited to the recommended max level, the battery tends to dry out gradually - or worse.
Gel batteries are the worst ones because of their higher internal resistance - they have an additional plastic membrane wrapper around the electrodes which is essentially non-conductive (these wrappers are needed to prevent sudden cell death caused by the slightest grid deformation and shorting in mobile environments).
What you can do is select higher capacity batteries for a higher total allowable charging rate.
Flat plate AGMs designed for stop/start applications, and thin plate/spiral lead/tin batteries offer higher charging rates per capacity unit.


On the other hand, it's not recommended to use a charger <1/10th of the 1C rate because the charger then might not switch back to float (the steady state current at the higher absorption voltage may then exceed the switch-over threshold which is usually found at ~10% of the specced maximum current).

cheers, Peter

Thanks to all for your comments.

As usual when you cannot get the answers from the people that actuallly supply the product themselves, the best source of information and theory is from forums like this one.


Ken

What ever happened to the olden days of camping and travelling.... when things were simple?

If you have the time and/or inclination, download this whitepaper -
http://www.victronenergy.com/upload/documents/ElectricityonBoard_rev8_july2004.pdf

on batteries. Quite in depth, but worth a read