is my battery large enough?

[Amunous (Michael)]

hello all i just bought a 26amp/hour 12v battery i was wondering if that would be sufficient enough to run a dew heater an argo navis and a cooling fan for maybe 1-2 nights?

im useless with electronics so i have no idea about amps and volts and things

[mswhin63 (Malcolm)]

From what I understand at the moment for dew heaters usually run at 45W which 3.75A @ 12V (I=P/V). On a 26AH rating battery this would be 6.93 hours. So in short only 3.47 hours per night at full power.

Condition that may influence the reading are switch on and off time for the dew heater, this would depend on a whole bunch of parimeters. I am designing my own dew heater to be automatically variable this could complicate matters as it would automatically switch on an off based on ambients (work in progress). I have never really checked the operation for other dew heaters though.

[bmitchell82 (Brendan)]

the easiest thing is to run a test yourself.

Get a inline amp meter and run everything that your going to power off it FLAT OUT. as much as you can draw current wise (amps)

take that number and do some simple calculations

Never run your SLA battery below 20% its just not good.

so 26 a-h - 20% is your theoretical amount of power

take your max current draw number you measured before.

26 divided by actual amps drawn. this will give you an approximation of how many hours it will last.

just some back ground info

Ah's means amps per hour
that means you can draw 26 amps for 1 hour or 1 amp for 26 hours.

hope that helps you out

[multiweb (Marc)]

Originally Posted by Amunous

hello all i just bought a 26amp/hour 12v battery i was wondering if that would be sufficient enough to run a dew heater an argo navis and a cooling fan for maybe 1-2 nights?

im useless with electronics so i have no idea about amps and volts and things

1 night might be ok but I think you're pushing it with 2. The dew heater will be the killer. Is it deep cycle?
In comparison I have a 75Ah deepcycle gel battery. I always make sure I don't go under 30% charge. I have the laptop, the dew heater, the G11 gemini, the camera. I can last a good 10h safely. I could push to two nights but I doubt it would do the battery any good.

[Amunous (Michael)]

yeah its a deep cycle battery.. i wouldnt be running the heater at full power all night just later on in the night when things start to cool off and the secondary starts to fog up. im only using 2 small bands as well one for the 1.25 eyepeice and a 4 inch for the secondary.
on second thought anyone have any idea how much power the argo uses ive looked on their site and cant find anything def about consumption. i could probably buy another battery to run the argo seperate. but ide need to know what to get to get a good 2-3 nights running from it. i can run the heater from the 26ah battery on and off.

here is a pic of the battery i just got

http://www.facebook.com/album.php?ai...&ref=fbx_album

[mswhin63 (Malcolm)]

I think overall you are gonna be pushing the limits to get 2 days out of 26AH. I run a total of 80AH and I dont have the dew heater yet. Without it I can get about 3 days but once I put a dew heater in I should get maybe 2 days.

Take Brendans idea and stack it together and measure the amp draw from the battery, that will give you a definative consumption to work out what you need.

In most of my constant current application I always add 10% on top of the reading, so if your final flat out reading is 60AH over 2 days then need to buy >66AH battery or batteries.

[xstream (John)]

30% DOD (Depth of Discharge) or 12.4V for a Deep Cycle battery is the norm.

[midnight (Darrin)]

Deep cycle and occassionally vrla batteries are used in solar and hence are designed for discharges well beyond 30% due to their thick wall plates. It is actually possible to discharge some more expensive types up to 85% without major damage if re-charged quickly - Yes this is true as we often do discharge tests in industry to prove the capacity to clients.

Temperature is the most important factor for astronomy as the battery is normally outside in the cold and its Ah rating is substantially reduced.

So for your 26Ah (assuming this rating is for a typical 20hr discharge rate) then you should size for temperatures around 0-8 degC so when we design installations at these temps, divide your Ah by 1.6 to get the 20hr capacity at temps around 0-8 deg C - ie 16Ah is your cool capacity (real capacity). Below 0 deg, use 1.8 reduction factor.

Then to get the most of this capacity, you can only discharge for 16A/20hr = 0.8A. If you pull more than this, then your "real" capacity drops yet again. Up to 35% for 2hr discharge. So if you want to discharge over 2hrs instead of 20hrs (which the manufacturer will claim) then your 16Ah capacity drops by 0.35x16 = 5.6Ah. Ie you now have only 16-5.6 = 10.4Ah capacity if discharging over 2hrs at temperatures from 0-8 deg C instead of 20hrs at 25 deg C.

I've probably confused you but if we say 26Ah starting and then using real situations (as suggested by Brendan) then your capacity is 10.4Ah/26Ah = 40% which the rest of guys here are suggesting 30% to factor in a safety margin .

Just remember in this example, your battery capacity is now only 10.4Ah and when it reaches this point, it is 100% discharged and not 40% based on the claimed rating! One more point, if using a deep cycle, you should temperature compensate when charging as a normal bulk charger will slowly damage it by reducing its capacity.

Just some background info as to how you get these figures.

Cheers,
Darrin...

[xstream (John)]

Originally Posted by midnight

Deep cycle and occassionally vrla batteries are used in solar and hence are designed for discharges well beyond 30% due to their thick wall plates. It is actually possible to discharge some more expensive types up to 85% without major damage if re-charged quickly - Yes this is true as we often do discharge tests in industry to prove the capacity to clients.

Yes this is true Darrin but you have neglected to add this will greatly reduce the amount of duty cycles your battery would be capable of if this was done on a regular basis.

[midnight (Darrin)]

John, it depends on the type of battery you install. Some of the more mainstream batteries you see on the market - yes regular "deep" cycling may affect its life but in my experience, a suitably selected battery should still perform well on deep cycling for many years (but expensive !!) provided it is recharged correctly and not left in a discharged state for too long.

Interestingly, I come across a lot of people on the road in caravans having bought a relatively cheap or unsuitable new deep cycle and found it is underperforming in only a few weeks. What happens is they are discharging the battery at night when your capacity is reduced due to temp and high discharge rates and then they stick on a standard battery charger & have truly begun to destroy the battery.

In the example in this thread is that the 26Ah battery could in fact have its "100%" capacity at only 10Ah and hence if it's discharged to 10Ah regularly in this environment and assuming it's one of the more "off the shelf" type deep cycles, then yes the life could be shortened significantly as you point out. However, incorrect charging after a deep discharge is one of main life shorteners for deep cycle because many chargers are not temperature compensated and not suitable for a deep cycle battery which has a specific charging curve.

I suppose it's like how much $$ do you have . Mount, scope, camera, then auto guiders, filters, field flatteners & oh - batteries... Ahhh there goes the budget for this year . Just don't tell the missus

Cheers,
Darrin...