Battery Chargers and Bulk Charge Rate

Submitted: Wednesday, May 02, 2012 at 20:05
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Hi all

I am sure I will be accused of asking a question about something that might have little effect on the overall outcome but as I have said before I am a technically curious person not always for reasons of whether it matters or not.

So to those that think this is a stupid question please do not reply and spoil what might be a good discussion by those that are interested in sharing their wealth of knowedge.


If it matters to the discussion let's use AGM deep cycle batteries.

Various multi stage chargers have different max current limits in the bulk charge phase of their operation, like 16A, 20A, 30A, etc.

Does the amount of current available in the bulk charge phase affect anything in terms of the effectiveness of the charge or health of the batteries, or does it simply affect the rate at which the batteries recharge to a point the charger will enter the next phase of the charge cycle?

Another way of asking the question would be "is it better for the longer term health of the batteries to have a lower current in the bulk phase and take longer than have a higher current bulk charge, or does it not matter" ?

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Reply By: Roughasguts - Wednesday, May 02, 2012 at 20:20

Wednesday, May 02, 2012 at 20:20
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.

AnswerID: 484833

Reply By: Eric Experience - Wednesday, May 02, 2012 at 20:56

Wednesday, May 02, 2012 at 20:56
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
AnswerID: 484840

Reply By: Ross M - Wednesday, May 02, 2012 at 20:58

Wednesday, May 02, 2012 at 20:58
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
AnswerID: 484841

Reply By: The Bantam - Wednesday, May 02, 2012 at 21:21

Wednesday, May 02, 2012 at 21:21
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 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 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.

AnswerID: 484845

Follow Up By: KenInPerth - Wednesday, May 02, 2012 at 23:47

Wednesday, May 02, 2012 at 23:47
Absolutely fantastic - thanks for the detail.

The consensus of 1/10 of the AH gives me some comfort in that I have perhaps inadvertently achieved a good result.

I currently have 2 x 120AH in parallel charged by a CTEK D250S dual which in the bulk phase is limited to 20A. This is stage 2 after a desulphation stage.

They have an option I was considering that would allow a higher charge rate until it drops to 20 A after which the D250 takes over to complete the smart charge.

I just checked my battery specs and it states initial charge 24A or less.


FollowupID: 760124

Follow Up By: TheMightyMoose - Thursday, May 03, 2012 at 13:02

Thursday, May 03, 2012 at 13:02
Very informative Bantam - thanks.
On a related issue, on my 3 stage charger I have the option to select one of 3 voltages (if I recall correctly 13.2, 13.5 & 13.8). Can you please explain the significance of these options and which is the most appropriate for non-AGM batteries (have one sealed and one with caps)? Thanks in advance.
FollowupID: 760162

Follow Up By: The Bantam - Thursday, May 03, 2012 at 18:38

Thursday, May 03, 2012 at 18:38
OK starting at the beginning.

Now the way batteres have developed, generally we can no longer rely on quite a few generalisations that we once held...we must consider each battery on it own...that said some generalisations are still reliable.

When we charge a battery we need to overcome the static voltage (the existing voltage in the battery) if we expect charge current to flow.

The higher the charge voltage is compared to the static voltage the more current will flow.

typical static voltage of a charged battery will be arround 12.5 Volts.

When we apply a charge a result of charge current flowing...... what was the static voltage will rise for various reasons we need to supply more voltage because of this, addition there will be resistive losses as mentioned by BV,....... we need more voltage to overcome those.

store that idea and move on

13.8 volts is generally considered to be the reliable steady charging voltage for all 12 volt nominal, lead acid batteries...still......current limitations aside.....

You can leave almost any lead acid battery on a steady 13.8 volt charge indefinitely as long as you keep up with that batteries service requierments.
Like if it is a screw top battery, you will need to replenish lost electrolite.

Some will cry and thow up their hands BUT......I insist that, other problems aside, you will fully charge any lead acid family battery with a steady 13.8 Volts, given sufficient time.

I have worked with a serviced many batteries float charged at 13.8 volts on an ongoing basis....most sealed batteries will operate float charged at 13.8 volts for their designed life span and retain their electrolite AT nominal room temperature 25C.

Lower voltages will result in less electrolite being lost from non sealed batteries and are more appropriate if sealed batteries are being charged at elivated temperatures.

The rate at which the battery charges will however slow particularly in the later stages.

Everything about lead acid batteries is effected by temperature.....they are happiest at arround 20-25C...heat is a major enemy.

While a battery may be perfectly happy indefinitely at a particular charge rate at 25C, raise that temperature to 45C or above, and continue that charge rate and you may damage the battery in several ways.

When a battery is charged at elivated tempertures, more heat is generated for a given charge it compounds.

Float charging a battery indefinitely at 45C compared to 20C may reduce its life expectation from 5 years to 1year.

SO I assume that the charge in question allows you to set the voltage of the final stage.

If you where keen and knew what you where doing, you would charge the battery and leave it on float charge.......after a period to allow settlement you would measure the temperature of the may then elect to reduce the charge voltage of the battery to maintain a tolerable temperature.

well in theory anyway.

OH, have a look at this link from Panasonic's does not cover everything, but it is an informative read...take particular note of the graphs involving temperature.

FollowupID: 760188

Reply By: Battery Value Pty Ltd - Wednesday, May 02, 2012 at 22:17

Wednesday, May 02, 2012 at 22:17
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
AnswerID: 484856

Follow Up By: KenInPerth - Thursday, May 03, 2012 at 10:02

Thursday, May 03, 2012 at 10:02
Thanks for this information

As per my reply to Bantam, I think I have inadvertently ended up with at least a reasonable solution.

FollowupID: 760141

Reply By: KenInPerth - Thursday, May 03, 2012 at 10:04

Thursday, May 03, 2012 at 10:04
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.

AnswerID: 484880

Reply By: olcoolone - Thursday, May 03, 2012 at 13:09

Thursday, May 03, 2012 at 13:09
What ever happened to the olden days of camping and travelling.... when things were simple?
AnswerID: 484893

Follow Up By: KenInPerth - Thursday, May 03, 2012 at 14:27

Thursday, May 03, 2012 at 14:27
I think that probably disappeared once cars started having on board computers and emission controls, and we needed TV and Microwaves and Washing Machines and Fridges and Hair Dryers and too much technology when we are on the road to have a relaxing holiday ....


FollowupID: 760166

Reply By: energy marty - Thursday, May 03, 2012 at 16:14

Thursday, May 03, 2012 at 16:14
If you have the time and/or inclination, download this whitepaper -

on batteries. Quite in depth, but worth a read
AnswerID: 484903

Follow Up By: KenInPerth - Thursday, May 03, 2012 at 16:20

Thursday, May 03, 2012 at 16:20

That will make some good bed time reading.

FollowupID: 760177

Follow Up By: Allan B (Member, SunCoast) - Thursday, May 03, 2012 at 17:27

Thursday, May 03, 2012 at 17:27
Wow Marty, a very comprehensive treatise. Written for marine application but useful for automobiles too.


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