How big of a battery do I need?

[paullad]

I'm new to all this so have mercy please. I need to power my laptop for 15 hours max between charges. My laptop uses about 20 watts. Is my math correct? 15x20=300 watt hrs/12 volts=25 amp hrs x 2 (max 50% drain) = 50 amp hr  + (how much for inverter draw?) = X amp hr battery needed. Thanks.

 

[Elbear1]

1 battery will be fine.

 

[highdesertranger]

is that going to be your only draw? no lights? radio? fan?

inverters inefficiencies depends on the inverter the better ones are better than 90% efficient my Xantrex claims 91%. my old cheap junk one was around 80%. some of your cheaper ones are lucky to be 70% efficient. some don't even tell you, they don't want you to know.

it always better to not run an inverter and to use a 12v adaptor.

highdesertranger

 

[paullad]

Thanks for that info hdr. Didn\'t know they made 12V adapters for laptops! Will look into that. I just looked it up and my Samlex is 87% efficient. I'll be part-timing in winter so fans won't be needed and lights will be solar.

 

[paullad]

Thanks but the question wasn't how many batteries but how big of a battery (amp hours). They come in all different sizes.

Elbear11 battery will be fine.

 

[Elbear1]

100AH agm or flooded. 50-75 amp lithium.

You on a budget?

 

[highdesertranger]

"15x20=300 watt hrs/12 volts=25 amp hrs x 2 (max 50% drain) = 50 amp hr + (how much for inverter draw?) = X amp hr battery needed."

up to this point you're ok,

"15x20=300 watt hrs/12 volts=25 amp hrs"

here's where you made a mistake,

"x 2 (max 50% drain) = 50 amp hr "

the max 50% drain gets subtracted from a lead acid battery not added to the load. you could add some to the load for inefficiencies, a little buffer, cloudy days, etc.

so you have a 25Ah load. like Elbear said a 100Ah lead acid would work fine. that would only use 50% of your available power or put another way it would take the battery down to 75%. you could take it down to 50% and be ok. that's where your 50% comes to play.

now for the big question,

how do you plan to charge this battery?

highdesertranger

 

[paullad]

That 50 ah gives me about 5 days of power. Since I'm going to be travelling a lot, I'm just going to use my vehicle.

 

[paullad]

100AH agm or flooded. 50-75 amp lithium.

Why are they different? I thought lithium batts should only be discharged 40-50% just like SLA.

You on a budget?

No, just cheap 

 

[Elbear1]

No they can be deep discharged

 

[highdesertranger]

yeah, lithium can be discharged to 20% no problem. highdesertranger

 

[B and C]

Lithium recharges much faster too.

 

[highdesertranger]

"Lithium recharges much faster too"

is that really true. I know my small things that use 18650 batteries take forever to charge. well not forever but many hours.

highdesertranger

 

[B and C]

From what I understand, they charge at a constant rate until full, no absorption or float. Whatever rate they can accept, they take that rate until full.

https://battlebornbatteries.com/charging-battleborn-lifepo4-batteries/

Edit to add: When my batteries are no longer up to the task, I am strongly considering lithium from battle born.

 

[Ticklebellly]

Lithium recharges much faster too.

They don't recharge faster.   They can accept a much higher charge current, compared to an equivalent AGM, hence can be recharged in less time than an AGM.

 

[B and C]

They don't recharge faster.   They can accept a much higher charge current, compared to an equivalent AGM, hence can be recharged in less time than an AGM.

Isn't that faster or am I missing something, everything else being equal?

 

[paullad]

No they can be deep discharged

I read one article and talked to the people at Jackery and both said you shouldn't discharge lithiums more than 50%. I talked to the Goalzero people and they said you can deep discharge. The Jackery people said the old lithium technology could be deep discharged but not the new technology. Guess I need to do a little more digging.

 

[Elbear1]

One of the big huge reasons to get them is you can discharge real low and recharge extremely fast.

You can discharge any battery all you want. It doesnt hurt them, perse. You get less life by doing so because the lead swells and slowly breaks apart while charging. Most people, especially with solar, sulfate lead acid long before they short them because of a factor seldom discussed; undercharging. Everyone talks about sizing a solar system as if the batteries are lithium - amps out = amps in. In reality you have to size solar with FLA to the bank size vs charge amps. In other words if you got a 200 amp bank you have to charge at least c12 (17 amps) + daily load. At least 400w and frankly 600w ideal or they will not reach absorption voltage and will undercharge every day. which will sulfate them.

So long story short throwing out 50% even with FLA would do many people good by going with a smaller bank more appropriate to their solar amp capacity.

You get to ignore all this hair pulling and we are all jealous lol

 

[Ticklebellly]

...... you shouldn't discharge lithiums more than 50%.  The Jackery people said the old lithium technology could be deep discharged but not the new technology.

Total misinformation.     I have set my BMS on the LiFePo4s to a low voltage cutout of about 12%.   I have been playing with high and low set points for about 4 years now.   If you are concerned, ask a manufacturer what are the high and low voltage settings maintained by the integral BMS of the battery of interest.   I'd also like to know what "new" technology they are talking about.

Time before recharge to 100% is the factor to be concerned about with Lead Acid.   The 50% myth re: AGMs is also just that.   I have an  8 year old AGM that gets discharged to 20% SOC regularly.    Deep cycle AGMs are designed to be deeply discharged.

 

[Ticklebellly]

Isn't that faster or am I missing something, everything else being equal?

Recharging faster and having a capability to accept a higher rate of charge (compared to Lead Acid) are not equivalent statements.