Battery System: British E-Class Subs

A question for any fans of WW1-vintage British E-class subs (operated from 1912 to 1922):

My question is, why would the voltmeters read in the 12-11 volts range, when the batteries operated at 120-volts? I found a book that has the following passage:

How’s the battery?’ ’ 12.10 and 12.06, sir,’ answered Seagrave. 'Hum, not bad. We’ve got enough amps. for another attack…


Let’s see, its three o’clock now. How’s the battery?’ ’ 11.85 and 11.81, sir,’ came the voice of the L.T.O., bending over the pilot-cell. ’ None too good, ’ went on the skipper. 'We’ll carry on on the series switch till five o’clock, we’ll be in 12 fathoms then, and sit on the bottom till dark. Ten o’clock will do it, I think.’


Underfoot [in the control room] as in the after compartment, was the great hundred-and-twenty volt battery which fed the hungry motors,

So if it was a 120-volt system, why did the voltmeters register the volts as 12-ish? Just because? Or was it a function of the way the cells were wired/grouped? The same book says the cells were 2.5 volts.

Other than this I haven’t found much on British WW1 sub battery systems. Here’s something about WW2 U.S. submarine battery systems:

American fleet submarines had two batteries, each composed of 126 cells. Each cell in a submarine battery produces from 1.06 volts when fully discharged, to 2.75 volts at the optimum output, so connecting the 126 cells in each battery in series gives a usable output of from about 210 to 350 volts…

I love these early British subs. Part of it is the amazing battles they got into. Sometimes the skippers seem more like fighter pilots than boat skippers. Part of the attraction it is that the systems on these early boats are, for the most part, easy to understand. They didn’t even have Christmas-tree indicator panels. To test for leaks they just charged the interior with compressed air, and if the pressure didn’t drop they were good to go!

Here in all its analog glory is a portion of the main switchboard on one of these boats:

Batteries in series add their potentials together.

so 13.75 volts/battery over 2.75 volts/cell is 5 cells/battery.

and 120 volts/bank over 13.some-odd volts/battery is 9 batteries/bank.

Then how many banks you can put in parallel will provide les amperes.

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The numbers refer to the specific gravity of the electrolyte, not voltage. It indicates the state of charge of the battery, a measure of the capacity remaining.


Thanks. That may be it. Elsewhere in the same book there is a reference to electrolyte density:

He and Seagrave conferred for a few minutes on the ‘care and maintenance’ of secondary batteries, and then the skipper turned to the voltmeter. 'Voltage 2.5. Yes, that’s all right. Densities 1248 and 1250.

But as you say, it might be that some of the engineers would say “1248” and others “12.48”, which would solve the mystery.

(By the way, the Royal Navy engineers back then were referred to as E.R.A.–Engine Room Artificers. I’ve never heard that term before.)

If you are interested in British submarine history in WW2, this book is outstanding. It deals a bit more with the personalities than the technical side but is a great read.
Sea Wolves - The extraordinary story of Britain’s WW2 submarines

If we are ever allowed to travel again, plan a visit to the he Royal Navy Submarine Museum at Gosport. It is small but it really gets the message across.

Back to the batteries, most would speak 1250 rather than 1 decimal 250 when talking to another crewman. A lead acid battery uses sulfuric acid and water as an electrolyte, a fully charged battery will have a specific gravity of 1.256, a deeply discharged battery will be somewhere at or below around 1.100. Discharging below about 1.150 or less than about 30 percent capacity is unhealthy for the cells. There are charts that show specific gravity vs charge corrected for temperature.

Battery maintenance on a diesel submarine is a very critical task. Engineers and electricians spend a large part of their days and nights tending the batteries. We used to do long overnighters “doing an equalizer” to restore each cell in the battery to equal voltage and charge. It was a complex mix of charge rate, time, temperature, voltage, and battery gassing called TVG - temperature/voltage/gassing which took many hours after completing a normal charge.

Note that when allocating fresh water, the batteries were a higher priority than the crew.


Most of us knew that it was a bastard of a job pushing a sub with flat batteries back to the surface hence there was a great deal of interest and concern in the condition of the batteries.
An ERA started as an apprentice in trade training, he never wore a sailors suit but wore fore and aft rig the same as a CPO only with black buttons not brass. After 4 years trade training, 5 for a Shipwright, he then started training for his unit tickets. As a ERA with a CPO rating he would be in charge of an engine room.
There were ordinance, electronic and a host of other artificers .


another book is “iron coffins” by a german U boat cmdr. ww2, newer tech but one of the best sub books out there. anyone notice most sub books seem to have been written by Americans?

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Another question I have about balancing electrical power needs on a WW1-era sub:

The British used the terms “group up” and “group down”. Group-up meant putting the boat’s two batteries in series. Group-down meant connecting them in parallel. When speed wasn’t needed the batteries would be grouped-down (paralleled). When attacking, the batteries would be grouped-up (in series).

My question is, if you wanted to keep the battery discharge rate at a minimum, wouldn’t you always keep the batteries in series?

Edward Beach in his book Run Silent, Run Deep said American subs did the same thing in WW2, only they just used the terms series and parallel. In making an attack they connected the batteries in series. Here’s what he wrote about a training attack on a S-class American boat:

Three thousand amperes to each of S-16’s two propeller shafts, six thousand total out of the main storage batteries, is a high rate of discharge in any league. For slow speeds the two main storage batteries are normally connected in parallel, and for high speed switched to series—thus doubling the voltage and halving the current for any given power requirement.

In neglecting to shift to series Jim was failing to get the maximum speed possible for the discharge rate and, in addition, was to no purpose risking damage to power cables and main motor armatures from the high current and the resulting heat. Our ship’s procedure was specified in the Engineering Orders: shift to series for everything over two thousand amperes per motor, and start with half the current…

Vainly I tried to catch his eye. He knew the score as well as I, as did everyone in S-boats for that matter, but somehow, in the stress of the moment he had completely forgotten. What was even harder to understand was the fact that he had, nevertheless, ordered a discharge rate far in excess of the allowable limit.

So it seems connecting the batteries in series gives you additional power and a low discharge rate. So why would you ever parallel? Is it a power curve thing, with amp-draw being the driving factor? The E=IR thing sometimes rivals E-MC2 for me sometimes. :slight_smile:

I read that book, very good. A family friend was a U boat commander in WW II. He was captured and went to a POW camp close to where we lived. Towards the last of the war the navy hired him and others to help bring captured subs into port for study. He got a lot of credit for that from the US government. When he was released from POW camp he returned to our home town to live as his family in Germany was all dead. Lots of interesting stories. The one I found most interesting was he bought fuel smuggled to him just outside Charleston SC by US German sympathizers or just greedy people, he was never sure. He said it was very expensive.

Parallel for endurance, series for sprint or escape.

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It seems likely to me that it’s a question of how much power is lost in speed control. Half voltage for low speeds and full voltage for high speeds.

My dad could have told you – he sailed with (then) CDR Beach on the maiden voyage of the postwar Trigger.


Took me a few minutes to see what you meant … as you are undoubtedly aware, the power absorbed by a propeller rises exponentially with RPM. If they wanted to speed up the boat, the power required would rise extremely high above that required for normal cruise speeds.

Think of the batteries as a fuel tank. If you push the power lever full ahead you are going to burn a tremendous amount of fuel to go at high speed. Back off on the power and your fuel will last a lot longer.

The cables supplying current to the motors are a finite size and have a finite resistance, they can carry a certain number of Amps without heating for a very long time but if the amperage is increased the heat produced by that resistance increases as well and at some point will exceed the ability of the surrounding air to cool the insulation.

If the boat was running with the batteries paralleled, the full capacity of the battery was available to produce X amount of power and go at Y speed until the battery was exhausted and no heating issues would arise. When the batteries were put in series, it was possible to drive the motors to much higher power, X plus some very large amount. But - the current did not remain the same as at “cruise power” it went up with the power produced by the motors and that caused the heating problems.

When starting the motors on series there was a short period when the electricians had to let the field current drop before ramping up again, that would be the “start with half the current”, it was kind of like shifting gears in a manual transmission. For a moment the RPM drops until the gas pedal goes down again. Once the maneuvering levers were arranged for series operation they would increase the current to obtain the higher power required. That also increased the battery current well above that used at cruise speeds.

The Amps/volts/power thing still applies and while higher voltage allows the motors to produce more than they could at slow speeds it also means the current rose to very high levels as well. Power is power no matter which side of the equation looks better and in this case that equation is P = I x V


Practically none was lost, those systems used good old fashioned mechanical contacts and there was essentially no conversion losses like we see in electronic switches and frequency controls. The guys in the maneuvering room shifted big levers and twisted big round knobs around to make the shafts spin the right way at the right speed.

Thanks much for the explanation above.

Here is a photo of the aft end of a British E-class submarine engine room. The engine control stations are to either side of the man sitting. You can see the backs of the EOTs up by the overhead. The two Vickers diesels were forward of the control stations. Under the deck are the electric motors. The photographer would be standing next to the stern torpedo tube,

Here is the main switchboard where they changed the connections for the batteries. The switchboard took up about half of the port side of the control room.The main batteries were located under the deck seen here.

Just aft of the panel is where the helms, hydroplane controls, periscopes, etc. were located. All controls were located in the control room. Nothing in the conning towers in these early boats.

The E-class subs are interesting in that in addition to the two bow torpedo tubes and the one stern torpedo tubes, there were “broadside tubes” that fired torpedoes to port and starboard. None of the WW1 British subs exist any more.

They mention a pilot cell. That’s one cell in the string that is actually monitored during normal operations.

Whether they were actually measuring the specific gravity or just inferring through a voltage reading on the pilot is hard to tell.

It does say they were right below the deck in the control room though so it’s possible they had the pilot cell rigged up with some kind of permanent hydrometer to take quick measurements on the pilot.

Let’s see, its three o’clock now. How’s the battery?’ ’ 11.85 and 11.81, sir,’ came the voice of the L.T.O., bending over the pilot-cell.

Maybe he was looking down at said hydrometer on the pilot cell mounted in the deck?

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That makes sense. Are there any other lead-acid battery systems that had such an arrangement?

I’ve never seen a hydrometer built into a battery but I did find this webpage that has pics of some old school batteries with them.


The electricians used a good old fashioned squeeze bulb hydrometer. The pilot cell was a cell located near the access hatch so it was easy to reach to take readings. Periodically they would test each cell. The record keeping was extensive and maintenance on the battery was continuous and not very appealing.

There was a cell voltmeter for remote reading of each cell.


great people still pass stuff like this around.

To take this further afield: The use of a colon or a ‘full stop’ (period) to denote time has been different in America or Britain or Continental Europe, especially since the time of the World Wars. Times (of London) style guide says:
times never write, eg, 6pm last night, 9am tomorrow morning; say 6 o’clock last night or (if the context allows) 6pm, or 9am tomorrow. Use a point in expressing continental time - 01.55, 14.00 etc .
So using the same analogy for the punctuation/grammar, the battery’s specific gravity would be spoken “eleven eighty-five and eleven eighty-one, sir,”

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