# How to fuse batteries connected in series?

Is it possible to wire each battery in a series to the next using a fuse wire, so that the individual batterie's max power output is not exceeded?

here is an illustration, suppose each battery had a maximum safe output of 60w, would it be possible to use 60w fuse wire in the place of the blue lines?

• Huh? You would put one fuse in series with the batteries. What does a 60W resistor have to do with anything? Post a schematic. – Matt Young May 19 '15 at 15:10
• Could you provide a circuit schematic? I don't quite get what you're saying. What is the rating of the fuses you are talking about? What is a 60W battery (i.e. is it the maximum output or the power in this circuit) – Dr Coconut May 19 '15 at 15:11
• What's a 60W battery? – Chu May 19 '15 at 15:11
• What's the resistor battery needs? – Eugene Sh. May 19 '15 at 15:17
• You should put one fuse in series with the full battery bank - not one per battery. And what is a "60 W battery"?? – Peter Bennett May 19 '15 at 15:46

## 2 Answers

The question contains some errors but I will try to answer what I think you are really asking. You're talking about fuses (fuse wire) so clearly you are interested in trying to protect the circuit.

When batteries are connected in series, the same current flows through all batteries. So you need a single fuse to protect them all.

Actually fuses are often used to protect the cable, and so fuses should be set to a lower rating than the current the cable can carry.

If you really are talking about resistors and putting them in series with the batteries: then standard formulae apply: power W = I.I.R (I squared R). V = I.R , W = V.I and all the other permutations of these formulae.

So you have to ensure that the power dissipated by the resistor does not exceed the maximum power rating for that resistor. If it does, the resistor will become excessively hot and burn out.

The voltage you know, the maximum current you know or you can calculate, and so you can calculate the power dissipated by the resistor.

You don't need fuse wire in each position. Fuse wire - of the correct rating - in one position is enough.

Let's spend a moment talking about fuse wire. Fuse wire is specified in Amps, as to how much current it can handle before it breaks. Fuse wire is not specified in W - watts.

Watts = volts x current. And the voltage doesn't matter, because it's an electrical conductor (made of metal), the voltage at any point along the fuse wire is the same (that's not entirely true, but good enough for discussion here).

So you're only interested in current when talking about fuse wire, which is measured in amps.

• Thank you! I wished to know, as Tesla fuse every battery in their cars, if it's possible for a 8p14s battery the same as a 14s8p battery? – com.prehensible May 19 '15 at 15:54
• I don't know how Tesla connect their car batteries up, it could be they're connecting the batteries in parallel to increase the current and increase the total ah (storage) capacity, in which case they would need a fuse connected to each separate battery. But a series connection of batteries - which yours is, the same current flows through each battery, so a single fuse is all you need, once that breaks (blows), the current through all the batteries stops flowing. – Dean May 19 '15 at 16:00
• Thanks, I understand. <<You don't need fuse wire in each position>>...? Tesla use a fuse wire on every battery, it's useful to save all 14k batteries if one of them becomes unstable? I am making prototype 18650 endcap connectors, without spot welding, i wanted a fuse in case one of them got too hot. – com.prehensible May 19 '15 at 16:03
• @ufomorace -- Fuses open when they are overcurrent, not when the battery gets too hot. You need a positive temperature coefficient device on each battery to open the circuit if it goes overtemp. "Protected" 18650's might have such an element built in. – Scott Seidman May 19 '15 at 16:13
• Thanks Scott! will see if panasonic and samsung and major 18650's co's have heat protection as standard. – com.prehensible May 19 '15 at 16:19

For series-connected batteries, you need only one fuse in series.

For parallel/series connected batteries (like in the Tesla), each group of series-connected batteries needs its own fuse.

Note that there are some conceptual issues with the question:

• Fuses are not rated in W, but in A (although a fuse will also have a maximum voltage rating).
• Batteries may have a maximum rated output current, which may be the 15A to which you refer, but it is rare to see the specification in W (maximum current times cell voltage).
• A common parameter is the Ahr rating which is (simplistically) a measure of the current capacity of a battery expressed as a nominal current draw (A) multiplied by the time in hours that the battery can deliver it (hr). You can convert this to energy (J) by multiplying by the voltage (and converting hr to s).

EDIT: The OP just added a diagram showing a different battery connection (from the Tesla?) which shows an extreme form of serial/parallel connection. Banks of individual cells are connected in parallel, and these banks are then connected in series. With this arrangement, you do indeed need a fuse on every battery. The advantage is that a failed battery (fuse) will remove only one battery from the pack and not (too much) affect the voltage/current characteristics of the overall pack. The first diagram does, however, show several cells in series - in this case only one fuse is required.

A WARNING: Connecting large numbers of lithium cells in series/parallel can achieve life-threatening voltages, and very high currents capable of doing serious damage to electronics and wires. High risk of fire too. Take care!

• Thanks! I have added a new picture, an 8p10s. i know that tesla puts a fuse on the individual plus terminals of all the batteries. I would like to fuse the battery as much as logically possible in the most rigorous degree to protect every individual battery, to know if it's possible to protect them individually. I understand what you mean about group of series. – com.prehensible May 19 '15 at 16:22
• evseupgrade.com/pic/?tesla-batt-module3 – com.prehensible May 19 '15 at 16:24