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 the DC wire size calculator for that, and note that a battery bank's main leads are usually the largest conductors in the whole build.
Common questions
- What size battery bank do I need?
- Work out your daily consumption in amp hours, then divide by the fraction of the battery you can actually use. A 100 amp hour daily draw needs about 125 amp hours of LiFePO4 or about 200 amp hours of AGM, before any allowance for cold weather or for days without charge. Sizing against the number on the label rather than the usable fraction is the most common way people end up with half the power they planned for.
- How deep can I discharge my battery?
- About 80% for LiFePO4 and about 50% for AGM, gel and flooded lead acid, if you want the cycle life the manufacturer advertises. Many lithium packs tolerate a full discharge at some cost to lifespan. The practical consequence is that a 100 amp hour lithium battery and a 100 amp hour AGM are not comparable products, because one gives you 80 usable amp hours and the other gives you 50.
- How much capacity do I lose in the cold?
- A LiFePO4 pack holds roughly 80% of its rated capacity at 0 °C and around 55% at −20 °C. More importantly, charging a lithium battery below freezing causes permanent damage through lithium plating, so a winter build needs either a heated battery or a battery management system that refuses the charge. Lead acid loses capacity in the cold too but is not damaged by being charged there.
- What is Peukert's law and does it apply to lithium?
- Peukert's law describes how a battery delivers less than its rated capacity when you discharge it quickly. It matters for lead acid, where a hard draw can cost you a noticeable fraction of the bank, and it is close to irrelevant for LiFePO4, which sits at an exponent of about 1.02. That difference is one of the real advantages of lithium and it is applied in the calculator rather than glossed over.
- Should I choose lithium or AGM?
- Lithium if you use the vehicle often, need the usable capacity, or are tight on weight and space. AGM if the up-front cost matters more than the running cost, or if the battery will sit in a place that goes below freezing while charging. Per usable amp hour lithium is generally cheaper across its life, because you get 80% of the capacity and several times the cycles.
- Does this size my solar and charging as well?
- No. This tool sizes the bank against a daily load. A bank that never gets fully recharged fails regardless of how well it is sized, so charging is a separate question. The one adjacent thing worth doing now is the cable: use the DC wire size calculator, because a bank's main leads are usually the largest conductors in the build.