Tools / Electrical
Battery bank, honestly sized
List what you run and for how long. This works out the energy off the battery, then applies the three derates most calculators skip: usable depth of discharge, temperature, and the Peukert effect. The answer is the capacity to buy, not the capacity you need.
# every step divides, and the order matters Wh/day = Σ(W × h)DC + Σ(W × h)AC / ηinv Ah = Wh/day × days / V C = Ah / (DoD × ftemp × fpeukert) # Peukert depends on C, so this iterates # to a fixed point.
The same load, every chemistry
Nameplate capacity is not what you get. Lead acid gives up half the label before you start shortening its life, so the same job needs a much bigger, heavier and more expensive bank than the sticker price suggests. Here is your load in all four.
| Chemistry | Usable | Bank to buy | Relative size | Cycles | Note |
|---|---|---|---|---|---|
| LiFePO4 ← | 80 % | 200 Ah | baseline | 3,000–6,000 | 80% is the conservative figure for long cycle life. Many packs tolerate 100%, at some cost to lifespan. |
| AGM | 50 % | 400 Ah | 1.60× | 400–1,000 | Sealed and maintenance free, but half the label is all you get. |
| Gel | 50 % | 400 Ah | 1.60× | 500–1,000 | Fussier about charge voltage than AGM, and slower to accept a charge. |
| Flooded lead acid | 50 % | 400 Ah | 1.60× | 300–700 | Cheapest per amp hour, needs topping up, vents hydrogen, and must stay upright. |
Why the label lies
A 100 Ah AGM does not give you 100 Ah. Take it much past half and you start measurably shortening its life, so the usable figure is nearer 50 Ah. A 100 Ah LiFePO4 will give up 80 Ah without complaint, and many will give the lot. That single difference is why a lead acid bank has to be roughly twice the size for the same job, and it is the reason people who size off the sticker end up with half the power they planned for.
Then two smaller effects stack on top. Cold batteries hold less, which matters if the bank lives in an uninsulated locker. And lead acid suffers the Peukert effect: the harder you work it, the less it delivers, so a 100 Ah cell rated over twenty hours gives about 84 Ah over ten and about 71 Ah over five. Lithium is nearly immune to both.
All three are applied above, in that order, and the step-by-step panel shows each division so you can check the arithmetic rather than trust it.
Cold weather is a safety question, not a performance one
Charging LiFePO4 below 0 °C plates metallic lithium onto the anode. It is cumulative, permanent, and it does not announce itself. If the bank can get near freezing you need a battery with a low-temperature charge cutoff, an internal heater, or the bank somewhere heated. Discharging in the cold is fine, just diminished.
Lead acid will accept a charge below freezing, which is one of the few arguments left in its favour, though it holds much less capacity down there and a discharged lead acid battery can freeze solid and split its case.
Published cold-weather capacity figures vary a lot between manufacturers. This calculator interpolates between documented anchor points and never claims more than rated capacity when warm, because overstating available capacity is the dangerous direction to be wrong in.
Chemistry reference
| Chemistry | Usable depth | Peukert | Cycles | At 0 °C | Charge below 0 °C |
|---|---|---|---|---|---|
| LiFePO4 | 80 % | 1.02 | 3,000–6,000 | 80 % | Never |
| AGM | 50 % | 1.10 | 400–1,000 | 80 % | Reduced rate |
| Gel | 50 % | 1.15 | 500–1,000 | 80 % | Reduced rate |
| Flooded lead acid | 50 % | 1.25 | 300–700 | 80 % | Reduced rate |
Once you know the bank size, the next question is the cable to it. Use theDC wire size calculator for that, and note that a battery bank's main leads are usually the largest conductors in the whole build.