diy solar

diy solar

Battery FAQ


Works in theory! Practice? That's something else
Sep 20, 2019
Key Largo
If you have a battery question and it's not a "beginner" question please try one of the battery boards.
Batteries play an important role in solar PV systems providing power
while the sun is down or during peak demand hours with excess grid charges.

During the day solar energy is converted by PV arrays and sent to a solar
charge controller (sometimes the controller may be inside the inverter).
This energy is used to recharge the battery, power devices within the
home, and export to the grid.

When sunlight or the grid isn't available or desirable to use, the
inverter converts battery power to AC for the home.

Video that compares battery technologies.

Question that will be in covered in this thread
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How does a 12V 100 Ah battery compare to a 1kWh battery?

Both are measures of how much energy the battery can supply for a given time. watts=amps x volts, so 12V * 100 amp hours = 1200 watt hours.
A kW is 1000 watts, so a 1 kWh battery is 1000 watt hours.

But, it's not quite that simple!
The actual capacity of a lead acid battery depends on how fast you pull power out. The faster it is withdrawn the less efficient it is.

For deep cycle batteries the standard Amp Hour rating is for 20 hours. The 20 hours is so the standard most battery labels don’t incorporate this data. The Amp Hour rating would mean, for example, that if a battery has a rating of 100AH @ 20 Hr rate, it can be discharged over 20 hours with a 5 amp load. If it has the rating of 200 AH, it can handle a 10 amp load for 20 hours.

Deep Cycle Battery datasheets will often show more detailed characteristics such as this Trojan:
TROJAN T-105 RE 225Ah 6V Battery
C-Rate2-Hr Rate5-Hr Rate10-Hr Rate20-Hr Rate48-Hr Rate72-Hr Rate100-Hr Rate
In this example you can see that when discharging the T-105 battery within 2 hours, it only stores 146 Ah and not 225 Ah.

All batteries are affected by temperature. For example this is from the Trojan SPRE 12 225's datasheet :
Lithium batteries are extremely sensitive to freezing temperaturs and can be damaged by charging at low temperatures. In extreme temperatures these batteries should be automatically disconnected or have a device to keep them warm.

Finally, most energy storage devices loose power over time. From the chart below you can see the Trojan SPRE 12 225 looses about 15% power per month.

So, which battery has more energy? You'll have to understand how you will use it and check the datasheets to know!

Use Cases & Battery Aging
Lead acid and Lithium batteries differ in another important way. Lead acid batteries lose capacity when the battery is stored at less then full charge and LFP batteries lose capacity when stored at full or no charge. See SoC degradation of LiFePO4 for more details.
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How do I compare the value of two batteries with different chemistries?

The value of a battery is somewhat objective because it depends on how you will use it. For example, lead batteries are much heavier than lithium batteries. There's also how much maintenance it takes, for example flooded-lead-acid batteries take a lot of special care.

The number of times a battery can be recharged is extremely important if cycling the batteries daily. However, if the batteries are only for emergency use and only cycled a few times per year, the number of cycles isn't that important.

How many times can a battery be cycled?
You can get this information from the battery vendor's data sheet. Let's take a look at the Trojan SPRE 12 225:

If you never discharge the battery below 50%, it can be recharged almost 2000 times! But at 50%, a 200Ah battery is only a 100 Ah battery.

50% is fairly standard for a lead-acid battery before the curve starts to really flatten out. Lithium batteries on the other hand can usually go 80 or 90% depth of discharge with 1000s of cycles. Here's experimental data for lithium-ion showing capacity loss as a function of charge and discharge bandwidth at a variety upper/lower SoC cutoffs at 1C.


The Math
The value of the batteries is the cost / kWh / DoD / number of cycles @ DoD

So how does the SPRE 12 225 compare to the BattleBorn 100 AH 12V ?
SPRE 12 225BattleBorn 100 AH 12V
Cost per unit$413$949
Useable Energy Watt hours225*12* 50% = 13501080
Cycle Life at DoD19003000
$/kWh as emergency backup$306$879
It looks like the SPRE 12 is as good or better than Lithium, right? Not so fast! After 3000 cycles the Battle Born batteries are guaranteed to be operating at least at 80%; those batteries can continue to be used. But, the lead acid batteries are essentially at end of life. Plus, the SPRE 12 are flooded lead acid batteries; skimp on the maintenance and they will have short life spans. Also, check the next post, lead acid batteries lose 10% more power while charging/discharging than LiFePO4, that's like loosing 1 in every 10 panels!

If you'd like to see how various factors affect a LiFePO4, check out a datasheet for them such as this one.


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How efficient are batteries at storing and releasing Energy?

You loose some energy when charging a battery, and some while taking it back out. How much depends on the chemistry type. Combining both the in and out efficiencies gives you the round-trip efficiency:

ChemistryRound-Trip Efficiency
Carbon AGM94%
Li Polymer98%
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What's a C rate?

Charge Rate,
or C-rate, is a measure of how fast a battery is charged or discharged relative to its maximum capacity, or going from empty to full or full to empty. The maximum C-Rate is the fastest charging time (most amps) without damaging the battery, it can be different for charge and discharge and is found on the datasheet.

An empty lead acid (LA) 12 volt battery would be down to 10.5 volts (V). Likewise, a 6V LA battery is considered empty at 5.25V. Generally, when you see a battery listed at a certain amp-hour (Ah) capacity, it is usually at a 20 hour C-rate. This means it takes 20 hours to fill or empty the battery at that rate. For example, a 225 Ah (Amp-Hours) Trojan T-105-RE battery at a C-20 rate would be 225 amp-hours ÷ 20 hours = 11.25 amps (A).
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How many Batteries do I need?

To answer this, you need to know your power consumption rate, how long you run it for, and much reserve you want for rainy days.

Let's say you look at your monthly power bill and it says you consume on average 892 kWh in 31 days. So, 892/31/24 = 1.2 kWh/hr

Discharging from a battery has inefficiencies, lead around .88 and lithium .96 to .98. So, if you're using Lithium it's 1.2/.96=1.25 kW/hr

With that number we can see the power consumed per day is 24 x 1.25 = 30 kWh.
If you want enough power for 3 days, you'd need 30 x 3 = 90 kWh.

As discussed in the post above, the power in batteries are rated at a standard temperature, the colder it is the less power they have.
You should check the actual datasheet for your batteries, but for typical lead acid it might be:
Temp FDe-rating Factor
So, with batteries expected to be at 40 to supply 10 kWh, with this data you'd multiply by 1.3 to see you would need 13 kWh of batteries.

A Tesla power wall is ~$700/kWh, so for 90 kWh it would cost $63,000.
This illustrates why it's so easy to get frustrated with batteries. Solar is cost effective, but batteries? Not so much right now. But prices are falling and new technologies are emerging.

The trick to minimizing your battery needs are to first reduce your power needs. For example, for emergency power you could turn your hot water tank off the breaker, they consume an average of 4 kWh/d.
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Designing a Battery Bank

Batteries come in discrete sizes: 18 Ah, 100 Ah, 200 Ah and so forth. When you need more stored energy than can fit in a single battery it is common to put batteries in series in strings, and to have multiple parallel strings. This works the same way as with solar panels in regards to voltages and currents, so if that's not clear to you start with What does it mean to have solar panels in parallel and series?

12, 24, 48, 300V?
Conversion efficiencies are typically higher with higher voltages and higher voltages for the same overall power demands mean lower current so less expensive wiring. But many people chose to stay at low voltages for compatibly with existing equipment.

How do I convert my Watt Power needs into a number of battery Ah?
You need 6 kWh/day and you want 3 days autonomy: 6000 x 3 = 18,000 Wh
You've selected lead acid batteries and you pick a conservative 40% Depth of Discharge: 18,000 / 0.4 = 45,000 Wh
You need that 6 kWh/d day when the ambient temperature will be 60F: 45,000 X 1.11 = 49,950 Wh.
Let use a 48V battery string. Watts = amps x volts, so amps = watts/volts: 49,950 / 48V = 1040 Ah

How do I design my Battery Bank?
When using lead-acid batteries it's best to minimize the number of parallel strings to 3 or less to maximize life-span. This is why you see low voltage lead acid batteries; it allows you to pack more energy storage into a single string without going over 12/24/48 volts.

There are many configurations that could work in the example above:
  • 4x 12V batteries rated at 1040 Ah
  • 8x 12V batteries in two strings of 4 all rated at 520 Ah
  • 16x 6V batteries in two strings of 8 rated at 520 Ah
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Wiring Batteries in Parallel

Here's a current representation of 4 batteries with the same internal resistance in parallel being charged with a 50 amp charger.
Notice anything wrong?
In order for the batteries to age at the same rate, they should be charged at the same rate. It's not to uncommon to see configurations like:


This is better, but still not ideal. How they're arranged to get the perfect balance is all about resistance. This is why you hear folks saying the cables should be the same gauge, the same length, etc.

Don't worry too much if that matches your existing configuration, the numbers in the illustration are using a thin guage and are a bit exaggerated to illustrate a point.

Gauge 0 wire has a resistance of about 100u/ft and new lead acid batteries have an internal resistance of around 50 millOhms going up to an ohm as they age. It's easy to build a circuit with tools like You can import the attached file Parallel1.txt into the program and play with the circuit to adjust things. In the circuit, batteries are represented in columns as a "voltage source" and "internal resistance". On the far right is a 2nd battery at a higher voltage acting as a charger. Horizontally are "resistors" representing the wires interconnecting the batteries.


Now that you've done your analysis and figured out the optimum cabling, did it look like this:




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Lithium Charging Parameters for four 3.2V cells in series (DIY 12V)

The manufacturer's recommendations should always be followed, but for DIYers when that's not available here's some guidance lift from one of Will's posts:

Set your solar charge controller to the LiFePO4 charge setting and do not bother charging to 90% SOC to extend the life of a LiFePO4 pack.

If you want your battery to last a long time, keep it in a cool location. Heat is the enemy! Also, try to size your battery so that the charge rate is less than .4C (which would be 40 amps for a 100ah battery). The larger your bank, the slower each individual battery is charged, and the longer your battery will last.

If you have manual control of your charge profile parameters, it is wise to use Victron's recommendation. This charge profile gives me 100% capacity in all of my tests and none of my cells voltages spike at all:

Absorption: 14.2V
Float: 13.5V
Equalization: Disabled
Temp Compensation: Disabled
Low Temp Cut-Off: 5 degrees Celsius

For the latest and best information check this LiFePO4 page with these charge profile parameters

LiFePO4 & Low Temperatures
You shouldn't charge LiFePO4 batteries below 32°F because it causes irreversible damage (video ref). This is why it's important to have a low-temperature cutoff on your BMS (many claim to have it, very few have it actually working). You won't run out of power in cold temperatures, your solar panels will still produce and you can draw from the battery. If you can't keep your batteries in a warm spot (e.g., indoors/buried) they can be kept in an insulated container and kept above 0°F by a battery heater. You shouldn't use an electric blanket without some mechanism (e.g., aluminum plate) to evenly distribute the heat as they have hot spots. You might also be able find "self-heating batteries" in the future.

LiFePO4 & High temperatures
Occasionally over 100° F isn't a problem, but too high (140°F) will also permanently destroy them (ref) and constantly running them over 100° can degrade your batteries 30% faster.
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Is it safe to discharge Lead Acid batteries to 20%?

It's not a matter of personal or battery safety; it's about battery economics and longevity. A lead acid battery won't explode even if you drain it dry. All that draining it to a low depth of discharge does is to dramatically shorten it's possible life span. The reason you typically see 45 to 60% depth of discharge (DoD) for lead is that is usually where the best economics lie for the average off-grid usage. Use the manufacturer's datasheet to understand life-cycle vs. DoD.

What's important is to understand your application. How many charge cycles you want out of a battery will enable you to understand the best DoD for you and allow you to compare different batteries at different costs in an apples-to-apples way.

Let's work an example. Say you want to cycle a battery everyday for 10 years; you need 3,650 cycles. Battery A costs $68.50 and has 500 cycles at 65% DoD. Battery B costs $1,000 and has 10,000 cycles at 90% DoD. Which is right for you?

Assuming the full DoD everyday, you need about two As for one B. Since 10,000 > 3650 you'd only ever need 1 so buying battery B will costs $1000. For Battery A, to get 3,650 cycles at the DoD you'd need 2x3650/500x$68.50 = $1000. So, in this case the price is the same.

As an exercise, which is better if Battery B gets 1000 cycles at 45% DoD? Which is better if you only need it for 50 cycles (e.g., emergency use over 10 years)?

Other factors:
  • The time value of money. One Battery A would last over a year, the money you didn't spend on Battery B can be making you money. Or, if you can't afford battery B, buying a Battery A now would hold you for a year so you could save up for it - and in that time Battery B's price might come down.
  • In addition to cycles being greatly affected by DoD, the actual power from lead chemistry depends on the current draw rate and ambient temperature, see Battery Power .
  • Weight, lead acid batteries are heavier. Not much savings if you hurt your back or crack your floor.
  • Battery maintenance/venting, FLAs in particular need a lot of care.
  • Low Temperature operation
  • Extra work in replacing more batteries (remember, they're heavy).
  • You generally get cash back for recycling lead batteries, you might have to pay to dispose of lithium.
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