Your meter reads 14.4V at idle. You hit a note and it dives to 11. Something is wrong — and the first thing almost everyone does is buy another battery.
Sometimes that is the answer. More often it is not, and you end up with an expensive pack that sags exactly as much as the old one because the actual problem was never the battery. Here is how to find it properly, in order, before you spend anything.
First: understand what sag actually is
Voltage does not drop because your battery is “weak” in some vague sense. It drops because current is flowing through resistance. Ohm’s law, and nothing more mysterious than that.
Every connection, every inch of cable, every crimp and every ground point has resistance. Push 800 amps through it and even tiny resistance produces a real voltage drop. A 0.01 ohm connection at 800A drops 8 volts. That is the whole problem in one sentence.
So the diagnostic question is not “is my battery good?” It is “where is my voltage disappearing to?”
The five checks, in order
1. Measure at the amplifier, not the battery
This one step solves a startling number of cases. Put your meter on the amplifier’s power and ground terminals — not the battery — while the system is playing hard.
If the battery holds 13.5V but the amp only sees 11V, your battery is fine. You have two volts disappearing in the wiring between them, and no battery on earth will fix that. If both read the same low number, then the problem is upstream and you carry on down this list.
2. Check your grounds. Then check them again.
Bad grounds cause more misdiagnosed battery problems than everything else combined. Grounds are invisible, they corrode quietly, and they look fine right up until you measure them.
Put your meter between the amplifier’s ground terminal and the battery negative post while the system is playing. You want under 0.1V. Anything above 0.3V is a problem. That reading is voltage being wasted in your ground path.
Look for paint under the ring terminal, a factory seam instead of solid chassis, a ground point that has been there through three builds, or a chassis ground doing work a big-three upgrade should be doing.
3. Torque every terminal
A lug that is finger-tight measures fine with the system off and falls apart under load. Thermal cycling loosens things. Go around every power and ground connection — battery posts, distribution blocks, amplifier terminals, alternator output — and actually tighten them.
While you are there, look at the crimps. A crimp that was done with pliers instead of a proper tool has resistance you cannot see. If a terminal is discoloured or the insulation near it has gone brittle, that connection has been getting hot, and heat means resistance.
4. Check your charging voltage against your chemistry
If you are running LTO at 14.4V, it is not sagging — it is doing exactly what an undercharged LTO pack does. LTO needs 15.2V minimum to reach rated output, and sustained undercharging damages it permanently.
Sodium ion runs happily from 13.8 to 15.5V, and LiFePO4 tops out at 14.6V. Charging outside the window does not just cost you performance, it costs you the pack. Our Support page has the full table.
5. Only now, look at the bank itself
If the wiring is solid, the grounds measure clean, the terminals are tight and your charging voltage is correct — now you can reasonably suspect capacity.
The question is discharge rate, not amp-hours. A pack with plenty of capacity can still be unable to deliver the current your system demands. Work it out both ways: total watts divided by the pack’s rated output, and total current divided by (Ah × C-rate). Whichever number is bigger is what you actually need.
The alternator question
At high power the alternator is not what keeps your voltage up during a note — the bank does that. The alternator’s job is recovery between hits. If you are at 30,000W and blaming a stock alternator for sag on the burp, you have the roles backwards. The alternator refills; the battery delivers.
What “normal” sag looks like
Some drop under load is physics, not a fault. What matters is how much and how fast it recovers.
- Healthy: drops a volt or so on a hard note, snaps back immediately when it ends
- Marginal: drops two volts and takes a second or two to recover — you are at the edge of your bank’s capability
- Problem: drops three or more volts, or recovers slowly, or the amps go into protect
- Wiring, not battery: big difference between what the battery reads and what the amp reads
The one that catches everyone
A brand new pack arrives reading around 11 volts and the owner assumes it is faulty.
It is not. Batteries ship at roughly 30% state of charge for transport safety. Bench-charge it up to your vehicle’s charging voltage before it goes in — 10A is plenty, there is no rush. Connecting a 30% pack straight to a running alternator is the fastest way to have a bad first day with a battery that was perfectly fine.
Still stuck?
Send us your pack model, your charging voltage, what the meter reads at the battery and what it reads at the amplifier under load. Those four numbers usually identify the problem in one reply — and we will tell you when it is a ground rather than something you need to buy from us.