Cramming all the power in a Battery.

[WarpedJester]

Hello again electro-wizards

So today i am trying to wrap my head around understand charging limitation, how much can you cram into a battery and how fast? what are the limitation and where do i look/how do i find these details.

THOUGHT EXPERIMENT:
This is just a though experiment to help me understand the charge process. the brands are not necessarily relevant other then for reference point for the sake of pointing out a data point. for focus is on the process. you can insert what ever brand/part number you like in your head.

Setup:
2x 12 Lithium Iron. batteries set up as a 24V cell. (lets pick Battleborn)
MPPT (we'll go with a or many victron 100/50 for this experiment)
100wh solar panels set up for 24v service.

The Experiment:
with the parts noted above, how many solar panels can you feed into those batteries before you are hitting a limit of input? i.e. 4 panels would be faster then 2 panels. what about 6? what about 60? is there a hard limit, is there a point of diminishing return? what and where do i look to understand where those lines are?

-Jester

 

[gnubie]

The limiting factor will likely be the output current rather than the input current since you gain amps converting the power from the panels down to battery voltage. Your maximum is about 1.4kW into the battery (28.8V x 50A) so, assuming paper rating is actually achieved, that'd be a total of 15 panels worth of production but it's more complex. Your array has to stay under the 100V limit of the controller and can't exceed 50 amps either but if you do have more than 50 amps available the controller will just limit at 50 amps so you can 'over panel' to some degree.

If you link to the 100W panel you are considering that'll, hopefully, provide the stats so array configuration can be determined.

 

[Forbisher]

Hello again electro-wizards

So today i am trying to wrap my head around understand charging limitation, how much can you cram into a battery and how fast? what are the limitation and where do i look/how do i find these details.

THOUGHT EXPERIMENT:
This is just a though experiment to help me understand the charge process. the brands are not necessarily relevant other then for reference point for the sake of pointing out a data point. for focus is on the process. you can insert what ever brand/part number you like in your head.

Setup:
2x 12 Lithium Iron. batteries set up as a 24V cell. (lets pick Battleborn)
MPPT (we'll go with a or many victron 100/50 for this experiment)
100wh solar panels set up for 24v service.

The Experiment:
with the parts noted above, how many solar panels can you feed into those batteries before you are hitting a limit of input? i.e. 4 panels would be faster then 2 panels. what about 6? what about 60? is there a hard limit, is there a point of diminishing return? what and where do i look to understand where those lines are?

-Jester

Two 12V 100Ah LFP batteries in series is 24V 100Ah
Battle Born batteries are 1C charging capable but 0.5C to 0.6C is recommended for longer life.
So you can charge at 50A to 60A
volts x amps = watts
24V x 50A = 1200 watts.

 

[John Frum]

Two 12V 100Ah LFP batteries in series is 24V 100Ah
Battle Born batteries are 1C charging capable but 0.5C to 0.6C is recommended for longer life.
So you can charge at 50A to 60A
volts x amps = watts
24V x 50A = 1200 watts.

To add to what @MBR is saying, lifepo4 batteries can draw enough current to hurt themselves so the charger has to be selected or configured with this in mind.

 

[Forbisher]

THOUGHT EXPERIMENT:
100wh solar panels set up for 24v service.

Your thoughts on "12V" 100w panels on 24V batteries are irrelevant to choosing a MPPT controller like a Victron 150 / 30 which has a max input of 150Volts open circuit.
Any panel can be series connected to add the combined Voc's to be safely less than 150V
100 watt panels are about 20Voc to 22Voc so you could safely connect up to 6 panels in Series at about 132Voc total.
Series strings of panels can be parallel connected which adds the amps of the combined panels at the same voltage

MPPTs have input voltage limits which they convert to output suitable 12V or 24V or 48V battery charging voltage.

 

[gnubie]

150/30? He mentioned a 100/50 model. 50A is fine for the Battleborn and sets the maximum power into the battery at about 1.4kW. Once he posts the panel specs we'll know what the array can look like and while it's likely they are 12V class panels you can get higher voltage 100W panels.

 

[WarpedJester]

thank you for the input. @gnubie again, this is just a thought experiment at the moment. i only pointed out battleborn and victron as examples of hardware in case someone needed a reference point or wanted to say "on something like victron, look at this label to fine xxx information.

For me personally, i am pretty sure i am going to go with battleborn batteries and a victron core due to the reviews and the connectivity/programmability. Panel wise, i am not sure. im still wrapping my head round poly vs mono, and one brand vs another but thats another topic for another post. i guess what i would ask is what am i looking at on a panel and what information do i need to make note of?

-Jester

 

[gnubie]

If you haven't settled on 100W panels unless you have a specific need for that physical size, shading, mounting area obstructions etc, IMO you'd be better off with larger panels.

Mono Vs poly largely comes down to watts/area, pricing wise a good poly is roughly the same as a good mono due to shipping and economies of scale factors. Polys are often a bit cheaper but IMO in the overall scheme of things not enough to make a difference in a domestic system. Quality for quality poly panels produce less power per area than the mono panels. Poly and mono have different temperature derates, poly being worse, and different light exposure performance, poly being better than mono in diffuse light. If you have the space the deficiencies of either compared to the other can be countered by adding more panels within the limits of the charge controller etc.

 

[ianganderton]

I think the C rate is the best way to look at this

Battle Born batteries are 1C charging capable but 0.5C to 0.6C is recommended for longer life.

For solar panels I think it depends on your useage case. My priority has been finding panels at the right dimensions (to fit neatly in the space available) at the right price. I’m finding that I can make almost anything work if I can get those bits right.

 

[WarpedJester]

Thank you again @gnubie and @ianganderton. while this is a focus on learning the ropes of calculations and what not, this is being driven by a upcoming project that i am starting which is a RV solar solution. I am lucky in that with this being a 31' Class C RV, i have a lot of roof space but as you can imagine, because its a RV, i do have to work the panels in and around vents and what not. So in this case, 100w panels are easier to tetris onto the roof.

@gnubie in regards to panels, i have been looking at the HQST 100w mono panels because they have gotten some positive reviews and notes that the build quality, scpes, and real world testing are just about clones of Renegy brands.

Since the topic has come up, i guess ill just ask here... what am i looking at on a panel spec sheet when planning what to buy?

-Jester

 

[gnubie]

You want to pay attention the the Voc, Vmp, and Imp mostly. With a MPPT charger the panel voltage must be a good margin higher than the battery voltage. Since you have 24V battery you should, IMO, be aiming for an array Vmp of 36V or higher. @MBR had it right with them being 12V class.

Voc 22.7, Vmp 19.1, Imp 7.89.

For a 100V Victron controller you could scrape in with 4 of those in series but that would not allow for cold temperature voltage rise so if that's a problem for you 3 in series for a Vmp of 57.3 at 7.89A max. You then add parallel sets of those 3-long-strings to get the watts you are after. You could also do 2 in series since the Vmp is still well above battery voltage at 38.2V but at the price of higher current. If you have more than 2 strings in parallel you should have series fuses between them and the common + wire, and optionally blocking diodes. A decent combiner box will have fuses. The fuse should be between 10 and 15A, 10 being high enough to prevent nuisance blowing and 15A being the max spec for the panel.

It all comes down to how you fit your desired array power into the Victron's 100V 50A ratings and not forgetting that you can put more than 50A on the Victron's input, within sane limits of course. It will just limit at 50A once it gets there for a flatter power curve across the day.

Regardless of array configuration the 50A limit on this model Victron's output together with battery voltage is what will limit your possible power production. 50A x 28.8V (Victron's default absorption voltage for lead acid) ~1.4kW.

*edit to add additional info*