Lead-acid, AGM or lithium: which battery should you buy?
All three work. The right one depends on how you use it, how cold it gets where it sits, and what you want to spend up front. Here's how to choose, and how to compare the prices you see in store.
Tool: Compare the prices you seeFirst: starting, marine or deep cycle?
Lead batteries all look alike from the outside. What matters is inside, and it's written on the label.
Starting battery (car, truck)
Lots of thin plates, built to give the starter a big burst of current for a few seconds. Drained often, it dies fast.
Avoid it for solar.
Dual-purpose “marine” battery
In between: it starts a boat engine and tolerates some discharges. Read the label: “Marine Starting” is a starting battery; “Marine Deep Cycle” is closer to deep cycle.
OK for light use, not every day.
Deep-cycle battery
Thick plates, built to be drained and recharged every day. 6 V golf-cart batteries are the typical example.
This is the one you want. The rest of this guide is about these.
Reading the label
A lead battery often shows several numbers. For solar, only one really matters: amp-hours.
- Ah (C20)Amp-hours
- Capacity over 20 hours, at low current. This is the number to use for solar and in our tools.
- RCReserve capacity
- A number of minutes: how long the full battery gives 25 A before dropping to 10.5 V. Made for cars (driving if the alternator fails). It is not Ah: never enter it as Ah.
- CCA / MCACranking amps
- The big current for 30 seconds to start an engine, at -18 °C (CCA) or 0 °C (MCA). It says nothing about the energy you can store.
- Group 24, 27, 31…Size
- The case size and terminal layout, not the capacity.
Your battery only shows reserve minutes?
Enter the reserve minutes printed on the label.
Real example: a battery rated at 185 reserve minutes is 105 Ah over 20 hours.
The short answer
Deep-cycle flooded lead-acid, if…
- the starting budget is tight, and weight doesn't matter (cabin, camp, shed);
- the battery sits in a ventilated spot where you can keep an eye on it;
- you're willing to top up distilled water now and then;
- you charge from the sun below 0 °C: it accepts charging in the cold.
AGM, if…
- you want maintenance-free lead that doesn't leak or need ventilation;
- you don't go often, or it's for backup during an outage;
- the battery has to charge in the cold;
- your charger or controller has no lithium setting.
Lithium (LiFePO4), if…
- you use the battery often: trailer, van, boat, a cabin lived in several days in a row;
- weight matters (about half as heavy);
- you want to use almost all the capacity, and charge fast from a generator;
- the battery stays warm, or it's a heated model, if you charge below 0 °C.
Side by side, for a 12 V 100 Ah battery
- Deep-cycle flooded
- 600 Wh usable · 1,200 Wh on the label
- AGM
- 600 Wh usable · 1,200 Wh on the label
- Lithium (LiFePO4)
- 1,150 Wh usable · 1,280 Wh on the label
| Deep-cycle flooded | AGM | Lithium (LiFePO4) | |
|---|---|---|---|
| Energy on the label | 1,200 Wh | 1,200 Wh | 1,280 Wh |
| Energy you can use | about 600 Wh (down to 50%) | about 600 Wh (down to 50%) | about 1,150 Wh (down to 10%) |
| Lifespan | often the longest of the lead types if well maintained | a few hundred cycles, or about 4 to 7 years | a few thousand cycles, often over 10 years |
| Weight | about 30 kg | about 30 kg | about 12 kg |
| Charging below 0 °C | yes, but slower | yes, but slower | no, except heated models |
| End of charge | slow: the last 20% takes hours | slow, a bit less than flooded | fast all the way |
| Maintenance | top up distilled water; it gives off gas while charging: ventilated spot | none | none |
Ballpark figures for deep-cycle batteries in good shape (not starting batteries). The manufacturer's spec sheet gives the real numbers for its model. 6 V golf-cart batteries, wired in pairs, are deep-cycle flooded lead-acid.
Compare the prices you see
Lithium costs more, but you can use almost twice as much of it. Enter prices for 12 V 100 Ah batteries (two or three types): we work out what each kWh you can really use costs you.
12 V 100 Ah · 0.6 kWh usable
— $ per usable kWh
12 V 100 Ah · 0.6 kWh usable
— $ per usable kWh
12 V 100 Ah · 1.15 kWh usable
— $ per usable kWh
Enter at least two prices to see the comparison.
We count 50% of the capacity for flooded and AGM (below that, they wear out fast) and 90% for lithium.
Questions to ask before you buy
- What charge and discharge current does the BMS allow?
- The BMS is a lithium battery's brain. If it only allows 50 A, it will cut out with a big inverter or charger.
- Does it charge below 0 °C?
- For lithium, ask whether it has built-in heating. Otherwise the battery refuses to charge when it's freezing.
- Is the warranty in years or in cycles?
- A 10-year warranty on paper is worth little if the seller disappears. A brand with service in your country is better.
- Do my charger and controller have a setting for this type?
- A lithium battery charged on a lead-acid setting fills poorly, or gets damaged.
Classic traps
- Comparing two batteries by their Ah without looking at the type: 100 Ah of lithium gives almost twice the usable energy of 100 Ah of lead-acid.
- Mixing a new battery with an old one, or two different types, in the same bank: the weaker one drags the other down.
- Leaving a lead-acid battery half empty all winter: it sulfates and loses capacity for good.
To calculate
Next step
Choosing solar panels
Rigid, flexible or bifacial, on the roof or on the ground, and a tool for the right angle for your location and season.