A couple of battery charging questions

Submitted: Tuesday, Jun 17, 2008 at 16:44
ThreadID: 58901 Views:3005 Replies:7 FollowUps:8
This Thread has been Archived
A bit dumb probably, but I don't know, so here goes. I have a 95 AMP hour battery & understand that I should not flatten it below about 50% of its capacity.
I have a charger that I bought from Jaycar, model MB 3612 which has a 12 AMP charging rate.
If I were to draw from the battery say 30 AMP hours, would the charger recharge it in 2.5 Hours? ( 30 divided by 12 ? )
Also, I believe you can get small portable type solar panels that are relatively cheap, but they also only have small output. How big a solar panel would you need to replenish the 30 AMP hours in one day. Thanks
Back Expand Un-Read 0 Moderator

Reply By: Member -Dodger - Tuesday, Jun 17, 2008 at 16:50

Tuesday, Jun 17, 2008 at 16:50
If you really use 30 amps per day then you would need a solar panel of at least 100 watts which is quite large.

I get away with a portable 80 Watt panel as when the vehicle is stationary then the panel supplies the Engel and when moving the vehicle supplies the fridg.
I used to have a handle on life, but it broke.

Cheers Dodg.

Lifetime Member
My Profile  Send Message

AnswerID: 310552

Reply By: Vivid Adventures - Tuesday, Jun 17, 2008 at 16:54

Tuesday, Jun 17, 2008 at 16:54
There will be efficiency issues which will slow it down the charging, and as the battery gets closer to capacity it will only accept charge at a lower rate, and the charger will switch down to a trickle feed.
The effect of drawing 30 AMPHours on the battery will depend on how quickly you draw it, and at what operating temperature it is working. Drawing 30 AMPHours really quickly might well flatten it.
How much power you get from the solar panel will, of course, depend on how much sun you get on the day, and how well you orient the panel.
AnswerID: 310553

Follow Up By: Member - barry F (NSW) - Tuesday, Jun 17, 2008 at 17:25

Tuesday, Jun 17, 2008 at 17:25
Thanks for that. My position is we want to sometimes stay on a non powed site for a period of three or four days. The 30 AMPS was arrived at from calculating the usage, as an example, of running six 15 W globes for a period of four hours. 6 X 15 divided by 12V X 4 Hrs = 30 AMPS ? I think that is the way it is calculated, but would be happy to be corrected.
If that is correct, then the 50% of our battery only gives us 47.5 AMP hours of which we would use up 30 AMP Hours on the first night.
We have A genny, since the Question how long to recharge it next day ( with consideration for others) & also why the solar alternative was asked. Even if a small solar unit only partially replaced previous night usage, it would extend our capacity a little.

FollowupID: 576566

Reply By: Mike Harding - Tuesday, Jun 17, 2008 at 17:21

Tuesday, Jun 17, 2008 at 17:21
>I have a 95 AMP hour battery & understand that I should not
>flatten it below about 50% of its capacity.

The more you discharge a battery the shorter its life will be.

If you can avoid discharging below (say) 20% you will get, pretty much the maximum, life from your battery. If you frequently discharge it to 50% you will get a short life span and if you discharge it below 25% capacity you may only get a handful of cycles from it.

I'm sure you appreciate these guidelines are not written in stone but they do represent the statistical norm - being familiar with the battery "experts" on this site I have no doubt there will be one who has discharged his 200Ah AGM to 0% every night for the past 10 years and "it's still going strong!" :)

As Andrew says; the Jaycar charger is a "constant voltage" device and will tail off as the battery charge approaches 100% so it may take (guessing) 4 hours to fully replace your 30Ah. You could go to a "constant current" charger but these are not recommended for lead acid batteries and may cause problems because they will drive the charging voltage up to 17V or so which is outside manufacturers spec. but you can get away with it if you're careful - I use one but I watch it closely and would not, generally, recommend one.

Solar panels: how long is a piece of string? An 80W panel ($500?) will probably provide somewhere between 3 and 6 amps of usable output for X sunshine hours per day, providing you can track the sun to some degree. In the Vic High Country in winter I suspect solar would be all but useless. However in The Alice in December it would be great.

Tell us more about where you go and what you need to run and we'll try to advise.

Mike Harding
AnswerID: 310560

Follow Up By: Member - barry F (NSW) - Tuesday, Jun 17, 2008 at 17:39

Tuesday, Jun 17, 2008 at 17:39
Thanks Mike, See my follow up above as to my intention. The "50%" as mentioned above was used as it seems to be the consensus of opinion on this site. I spoke with Battery world today, and was advised that it was best not to discharge a deep cycle battery below 30% of its capacity. We only want to run a few lights as outlined & maybe a TV for a few hours.
I want to avoid the use of a Genny for all the obvious reasons, hence I wondered whether a bit of a top up each day from solar, provided it was not bulky & expensive was an option.
FollowupID: 576569

Follow Up By: Mike Harding - Tuesday, Jun 17, 2008 at 18:51

Tuesday, Jun 17, 2008 at 18:51
Hi Barry

6 x 15W lamps is a lot of light for a campsite. You have a caravan I assume and I usually rough camp in deep bush with a swag but I get by, quite happily, with either one 12W long life fluro or, preferably, my Coleman dual fuel lamp, or just the firelight (best of all :) I use my lighting to read by or play Backgammon etc without issues - are you _sure_ you need 90W of light?

Mike Harding

PS. Your calculations are correct as is the advice from Battery World.
FollowupID: 576593

Follow Up By: Member - barry F (NSW) - Tuesday, Jun 17, 2008 at 19:13

Tuesday, Jun 17, 2008 at 19:13
Mike, yes we have a caravan & you are talking to the "converted" when it comes to the warmth & ambience of the light from a camp fire!! Could not agree more.
I used the the 6X15 W lamps as an example in order that if my method of calculating power usage from a battery was wrong, then I could be corrected. Just a mathematical thing & looking for confirmation that my method of calculation is in fact correct.
My aim on posting the question was to establish what in reality I could expect from the battery given the Wattage of various things being run & the possibly of "topping it up" (not necessarily fully recharging it) the next day Via using solar power. Your input is much appreciated as is every bodies. I 'am not a Genny fan & would only use one if we were in an isolated place, well away, not even close, not within "whooppee" of other campers etc. Cheers & thanks.
FollowupID: 576604

Reply By: Mainey (wa) - Tuesday, Jun 17, 2008 at 21:05

Tuesday, Jun 17, 2008 at 21:05
If you are running down the battery by a fridge, then it will be running more during the day than at night.

If you bought a 100 watt solar panel it would supply ~6 Amps during the nice sunny periods of the day, the fridge would be using less than is produced by the solar panel, so the fridge would be running and the battery would also be being charged at the same time.

Top Steca picture shows my fridge is drawing 7.8 Amps (obviously not Engel or Waeco)
The white Amp gauge shows 0.5 Amps is also charging the (13.8 Volt) battery

Total Amps supplied is only 8.3 out of ~12 Amps available, because the battery is almost always fully charged.

Image Could Not Be Found

This battery system is only dis-charged during the night and when no power is received at Solar system regulator.
However, when a fridge is not running during the day ALL power required is supplied to the battery system by the solar regulator, your fridge possibly runs on about a 50% duty cycle so you will maintain a fully charged battery during the day.

This system avoids the battery from being discharged to the drastic numbers talked about.

Mainey . . .
AnswerID: 310635

Reply By: oldtrack123 - Tuesday, Jun 17, 2008 at 21:35

Tuesday, Jun 17, 2008 at 21:35
Hi Barry
#1 yes your method of calculating use is correct, watts divided by volts = amps drawn,
#2 multipy amps drawn per item by estimated hrs of use = amphrs used per item
#3 add amphrs for all items= total amp hrs used per day
.#4 charging ineffiency means you would have to put in approx 25to 50 % more in than you took out . take out 30 ahrs put back up to 45ahs
#5 as others have said rate of draw can affect available amp hrs
#6 charging at 1or 2 aps will not do much to charge, For long life of deep cycle baaterries recommended charging current is 20% of amp hr capacity ,ie 4amp for 80 amp hr battery,but higher rates can be used.
#7 a 80w solar panel will give 4 to 4.5 amps output on a bright sunny day[ sometimes up to 5amps under very high light conditions]
You can generally assume 6hrs @ 4to4.5 amps if you have panel tilted & facing the sun so that it is at 90Degrees to sun, ie 24 to 28 amphrs input.You can gain a little more if you track the sun all day
AnswerID: 310640

Reply By: Sand Man (SA) - Wednesday, Jun 18, 2008 at 09:31

Wednesday, Jun 18, 2008 at 09:31

The advice you got from Battery World was quite practical and good on them.

For occasional use, running the battery down to 30% (about 11.6 volts) is a more realistic practise. That is why fixed low voltage cutout devices are set to this voltage.

You should get at least 2 days of use, unless the fridge is very hungry.

What you then need to determine is a practical way of replenishing the energy in the battery. Some people do this via a solar panel, which can be connected permanently. The solar panel has to be big enough to put back as much and preferably more amps than you are taking out.
Alternatives to a solar panel is either a generator, (not always socially acceptable) or via the vehicle alternator, preferably taking the vehicle (and fridge) for a drive.

I have been with people who run their vehicle stationary for half an hour, but that doesn't put back anywhere near enough amperage to last that long.

You haven't stated how big your fridge is, or what the current draw is, but a solar panel of even 40 watts may be enough, without over capitalising on an investment. A 40 watt panel will put in around 2 amps continuously for several hours each day. Sometimes more, sometimes less, depending on sunlight etc.


I'm diagonally parked in a parallel Universe!

My Profile  My Blog  My Position  Send Message

AnswerID: 310721

Follow Up By: Member - barry F (NSW) - Wednesday, Jun 18, 2008 at 10:03

Wednesday, Jun 18, 2008 at 10:03
Thanks Bill & Mainey, Our Frig runs on gas. I just want to run a few lights & and My wife will probably use the TV a bit. The light bulbs are only 15 W & the Telly is 60 W. There is a water pump that would get very minimal use.
FollowupID: 576705

Reply By: tuck 01 - Wednesday, Jun 18, 2008 at 10:17

Wednesday, Jun 18, 2008 at 10:17
As you recently benefitted from one of my "dumb" questions, I have learned a lot from the response to this question.
Thanks for asking.
What worries me is that if you and I find ourselves set up in a camp remotely together, we could be in real trouble unless we can log on to the forum for advice.
Happy planning.
AnswerID: 310723

Follow Up By: Member - barry F (NSW) - Wednesday, Jun 18, 2008 at 14:47

Wednesday, Jun 18, 2008 at 14:47
No No No Jeff, all you've go to do is to take your wife with you, or if you haven't got one of those, take your woman, beacause they not only know everything, they are also ALWAYS right, no matter what!! LOL
FollowupID: 576739

Follow Up By: Dunaruna - Wednesday, Jun 18, 2008 at 17:38

Wednesday, Jun 18, 2008 at 17:38
I could rent out my teenage son, he knows absolutely everything about anything.....................lol
FollowupID: 576778

Follow Up By: Member - barry F (NSW) - Wednesday, Jun 18, 2008 at 18:03

Wednesday, Jun 18, 2008 at 18:03
Thanks for the offer Dunaruna, but with one wife, three daughters & a son that you have just described, I will decline.
But thanks anyway!! LOL & cheers.
FollowupID: 576784

Sponsored Links