How Many Watts x Amp Hours with Lithium?

[VanForNow]

When planning a solar install with FLA batteries, it is often recommended to have twice as many watts in the solar panels than amp hours in the batteries.

Does this ratio change when planning a lithium battery install?

 

[jimindenver]

it doesn't matter how many batteries you have if you do not produce more power than you use. The 2 times watts (or better) comes into play when you deplete the bank and need to bring it back up quickly. That's when you need enough power to push past the resistance of the bank while charging.

 

[DLTooley]

That’s a rough rule of thumb. Your actual usage and solar weather are worth figuring out. Make sure you estimate on useable watt hours and use a valid apples to apples measure.

 

[VanForNow]

Thanks, Jim, but since FLA batteries cannot be drawn down below 50% without damaging them, I was wondering if the more "forgiving" nature of lithium batteries means that you can get by with fewer amp hours with lithium than when buying FLAs?

 

[Trebor English]

Thanks, Jim, but since FLA batteries cannot be drawn down below 50% without damaging them, I was wondering if the more "forgiving" nature of lithium batteries means that you can get by with fewer amp hours with lithium than when buying FLAs?

Lithium batteries are absolutely not forgiving.  They are typically deployed with a battery management system, BMS, to disconnect them automatically to avoid undercharge and overcharge.  One deep discharge, headlights left on over the weekend, will hurt a lead acid battery but not kill it.  A similar total discharge of a lithium will kill it.  That's why they have a battery management system.

The 50% discharge limit recommendation is to give better performance.  If you cycle them too deeply they are less likely to get a 100% recharge.  The people screw it up.  Repeated 80% recharges decrease the battery capacity.   That leads to disappointment and short battery life.  Repeated 80% discharges followed by full 100% recharges are much less wear.  

Yes, you can calculate based on discharging lithium batteries by 80% to a 20% state of charge.  To calculate that precisely you need to predict your use of electricity and the solar charging weather.  How many days do you expect the battery to provide?

 

[VanForNow]

Lithium batteries are absolutely not forgiving.... How many days do you expect the battery to provide?

From what I have read (which is limited), a lithium battery is protected as long as the BMS is functioning. But do FLAs have similar protection to prevent them from excessive discharge? As to how many days I expect the battery to provide, I am presently seeking basic information and do not have yet have specific parameters in mind.

 

[Trebor English]

Flooded lead acid are sufficiently durable and resilient to be able to work without a BMS.  That said, many people have a low voltage disconnect.  My charge controller has a load output with programmable voltage.  When my battery voltage goes down to 12.2 my LED lights go off so I am warned and can take action turn off other stuff.  

It is common for a fridge to be the biggest part of your electricity use.  The actual consumption is highly variable making planning difficult.

 

[Ticklebellly]

........ but since FLA batteries cannot be drawn down below 50% without damaging them, I was wondering if the more "forgiving" nature of lithium batteries means that you can get by with fewer amp hours with lithium than when buying FLAs?

The 50% thing is common lore not supported in actual use.      I am currently playing with an off grid situation where a 110 AH AGM gets drawn down to 11.8 Volts every second day.   The battery is ex UPS and I have had it 6.5 years.

The no more than 50% anxiety is implicated discussion of panel watts feeding AH because another rule of thumb has a goal of seeing your AGM or deep cycle battery bank going to float in the mid to late morning in good charging weather.   In marginal weather, you get the afternoon as extra time to get the battery back up to full charge.   An average daily draw that allows for float by mid morning also suggests your battery bank has a bit of capacity to deal with bad weather.

A common assertion regarding Lithium is that 50 Ah Li can be considered equivalent to 100 Ah AGM.   The reasoning is that you can take Li down to 10% but need to restrict draw on an AGM to 50%.    Following this line of thinking says equal panel watt numbers to Li batt AH numbers is a reasonable way to determine sizing ratio.

If you have an existing panel to AGM setup, buying half the AH in Li would be less hurt to the bank account, too.

I am currently helping a friend and have installed a 120W Semi flex panel and a DIY LiFePo4 50 AH battery with a low voltage cutoff set at 11.8. Only a couple of months but she is not having any problems.

 

[bigskybob]

50 ah is not much capacity regardless of which battery you have.That's only 5 amps ac for an hour.

 

[blars]

With lead-acid, you should have enough solar to recharge to full every day, even when there is some clouds. LiFePO does not need to be fully charged, and can handle long periods of partial charge as long as you don't get below 10-20%. Also, they are more efficient so you can get by with fewer panels. Make sure you have at least enough panels to replace what you use on average.

 

[Ticklebellly]

.......5 amps ac for an hour.

????????

 

[AuricTech]

To clarify, 50 AH at 12 Volts direct current is the same number of Watt-hours as 5 AH at 120 Volts alternating current.

 

[Groovy2]

I have been using Lithium in van for close to 2 years daily -
I built my system from scratch using Calb 100ah Lifepo4 cells-
Victron solar charger and Victron Low voltage cutoff

My battery actually will put out 120 ah - (listed as 100 ah battery)
Calb Oversizes their cells by 20% of listed AH rating -

Discharging Lifepo4 cells down past 2.8 V( Cell voltage each) is not good for them but
will not instantly kill them as seem to be the commonly thought -

Discharging cells to 3.0 volts is OK but there is not really much power
left in battery at that point and battery voltage falls off quickly- and there
is only a Tiny amount of power left at that point-

Voltage in Lifepo4 Batteries dose not drop steadily like lead batteries do-
Lifepo4 4 cell battery will stay at 13.2 volts for Hours of use -

a Victron Low Voltage power Cutoff switch is added into system and will cutoff
power from battery at voltage you set - this cutoff switch costs $50

works well -

 

[tripper]

Lithiums should not be allowed to drop below 12.6 volts (resting voltage) that is about the 20%, after that they will drop voltage very fast until the BMS cuts the power. The problem with the bms is it's power cut out is too low and some damage to the lithium battery will occur before the cutout limit (not much, but it does damage it). Flooded lead acids 12 volts is the lowest resting voltage you should let them get to, but even that is harsh on them and they will live a shortened life.

The voltages will show a bit lower under heavy loads, but I find disconnecting all loads and letting the battery sit for 5 minutes before taking the volt reading works very well for me.

I highly recommend 3 x the solar wattage to AH of the battery for a year round all areas of the country traveler. 2 x works fine in sunny Arizona, but will leave you conserving power a lot in many climates.

I have been running both lead acid and lithium in my rig now for a while (2 systems) and found 3x to be the minimum for almost no worry charging all the time day after day, especially with lithium as even many rainy cloudy days you can get 60-90 minutes of sun in cloud breaks and this will get the batteries up a lot.

 

[nemoskull]

storage capacity doesnt really matter with lithium. the reason to size your FLA to solar is to avoid leaving the batteries discharged. lithium doesnt have that problem. so the only thing that matters is charging rate.
so something along the lines of 1c max charging, i.e. 400 ah battery bank no more than 400a charging max.
tho keep in mind this is a genreal rule of thumb with LIFEPO4. other chemistry vary. turningy nano tech, can charge at 5c. nissan leaf doesnt really list, but there only rated at 0.3c discharge, so i assume charging is the same.